patch 8.2.0800: errors from failing test are unclear

Problem:    Errors from failing test are unclear.
Solution:   Include text where parsing failed.
diff --git a/src/json.c b/src/json.c
index ec3355e..2720abf 100644
--- a/src/json.c
+++ b/src/json.c
@@ -20,6 +20,8 @@
 
 static int json_encode_item(garray_T *gap, typval_T *val, int copyID, int options);
 
+static char e_json_error[] = N_("E491: json decode error at '%s'");
+
 /*
  * Encode "val" into a JSON format string.
  * The result is added to "gap"
@@ -740,7 +742,7 @@
 			retval = json_decode_string(reader, cur_item, *p);
 		    else
 		    {
-			emsg(_(e_invarg));
+			semsg(_(e_json_error), p);
 			retval = FAIL;
 		    }
 		    break;
@@ -748,7 +750,7 @@
 		case ',': // comma: empty item
 		    if ((options & JSON_JS) == 0)
 		    {
-			emsg(_(e_invarg));
+			semsg(_(e_json_error), p);
 			retval = FAIL;
 			break;
 		    }
@@ -778,7 +780,7 @@
 			    }
 			    if (!VIM_ISDIGIT(*sp))
 			    {
-				emsg(_(e_invarg));
+				semsg(_(e_json_error), p);
 				retval = FAIL;
 				break;
 			    }
@@ -809,7 +811,7 @@
 				    &nr, NULL, 0, TRUE);
 			    if (len == 0)
 			    {
-				emsg(_(e_invarg));
+				semsg(_(e_json_error), p);
 				retval = FAIL;
 				goto theend;
 			    }
@@ -962,7 +964,7 @@
 			retval = MAYBE;
 		    else
 		    {
-			emsg(_(e_invarg));
+			semsg(_(e_json_error), p);
 			retval = FAIL;
 		    }
 		    goto theend;
@@ -980,7 +982,7 @@
 			retval = MAYBE;
 		    else
 		    {
-			emsg(_(e_invarg));
+			semsg(_(e_json_error), p);
 			retval = FAIL;
 		    }
 		    goto theend;
@@ -1036,7 +1038,7 @@
 			retval = MAYBE;
 		    else
 		    {
-			emsg(_(e_invarg));
+			semsg(_(e_json_error), p);
 			retval = FAIL;
 		    }
 		    goto theend;
@@ -1055,7 +1057,7 @@
 	res->v_type = VAR_SPECIAL;
 	res->vval.v_number = VVAL_NONE;
     }
-    emsg(_(e_invarg));
+    semsg(_(e_json_error), p);
 
 theend:
     ga_clear(&stack);
@@ -1079,7 +1081,7 @@
     if (ret != OK)
     {
 	if (ret == MAYBE)
-	    emsg(_(e_invarg));
+	    semsg(_(e_json_error), reader->js_buf);
 	return FAIL;
     }
     json_skip_white(reader);
diff --git a/src/testdir/test_json.vim b/src/testdir/test_json.vim
index 94481d9..6c11ace 100644
--- a/src/testdir/test_json.vim
+++ b/src/testdir/test_json.vim
@@ -148,36 +148,36 @@
   call assert_fails("call json_decode('{\"\": \"ok\", \"\": \"bad\"}')", 'E938:')
 
   call assert_equal({'n': 1}, json_decode('{"n":1,}'))
-  call assert_fails("call json_decode(\"{'n':'1',}\")", 'E474:')
-  call assert_fails("call json_decode(\"'n'\")", 'E474:')
+  call assert_fails("call json_decode(\"{'n':'1',}\")", 'E491:')
+  call assert_fails("call json_decode(\"'n'\")", 'E491:')
 
-  call assert_fails('call json_decode("\"")', "E474:")
-  call assert_fails('call json_decode("blah")', "E474:")
+  call assert_fails('call json_decode("\"")', "E491:")
+  call assert_fails('call json_decode("blah")', "E491:")
   call assert_fails('call json_decode("true blah")', "E488:")
-  call assert_fails('call json_decode("<foobar>")', "E474:")
+  call assert_fails('call json_decode("<foobar>")', "E491:")
   call assert_fails('call json_decode("{\"a\":1,\"a\":2}")', "E938:")
 
-  call assert_fails('call json_decode("{")', "E474:")
-  call assert_fails('call json_decode("{foobar}")', "E474:")
-  call assert_fails('call json_decode("{\"n\",")', "E474:")
-  call assert_fails('call json_decode("{\"n\":")', "E474:")
-  call assert_fails('call json_decode("{\"n\":1")', "E474:")
-  call assert_fails('call json_decode("{\"n\":1,")', "E474:")
-  call assert_fails('call json_decode("{\"n\",1}")', "E474:")
-  call assert_fails('call json_decode("{-}")', "E474:")
+  call assert_fails('call json_decode("{")', "E491:")
+  call assert_fails('call json_decode("{foobar}")', "E491:")
+  call assert_fails('call json_decode("{\"n\",")', "E491:")
+  call assert_fails('call json_decode("{\"n\":")', "E491:")
+  call assert_fails('call json_decode("{\"n\":1")', "E491:")
+  call assert_fails('call json_decode("{\"n\":1,")', "E491:")
+  call assert_fails('call json_decode("{\"n\",1}")', "E491:")
+  call assert_fails('call json_decode("{-}")', "E491:")
 
-  call assert_fails('call json_decode("[foobar]")', "E474:")
-  call assert_fails('call json_decode("[")', "E474:")
-  call assert_fails('call json_decode("[1")', "E474:")
-  call assert_fails('call json_decode("[1,")', "E474:")
-  call assert_fails('call json_decode("[1 2]")', "E474:")
+  call assert_fails('call json_decode("[foobar]")', "E491:")
+  call assert_fails('call json_decode("[")', "E491:")
+  call assert_fails('call json_decode("[1")', "E491:")
+  call assert_fails('call json_decode("[1,")', "E491:")
+  call assert_fails('call json_decode("[1 2]")', "E491:")
 
-  call assert_fails('call json_decode("[1,,2]")', "E474:")
+  call assert_fails('call json_decode("[1,,2]")', "E491:")
 
-  call assert_fails('call json_decode("{{}:42}")', "E474:")
-  call assert_fails('call json_decode("{[]:42}")', "E474:")
+  call assert_fails('call json_decode("{{}:42}")', "E491:")
+  call assert_fails('call json_decode("{[]:42}")', "E491:")
 
-  call assert_fails('call json_decode("\"\\u111Z\"")', 'E474:')
+  call assert_fails('call json_decode("\"\\u111Z\"")', 'E491:')
   call assert_equal('[😂]', json_decode('"[\uD83D\uDE02]"'))
   call assert_equal('a😂b', json_decode('"a\uD83D\uDE02b"'))
 endfunc
diff --git a/src/version.c b/src/version.c
index f79ad75..ede8ad1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    800,
+/**/
     799,
 /**/
     798,