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/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