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);