patch 8.2.1183: assert_fails() checks the last error message

Problem:    assert_fails() checks the last error message.
Solution:   Check the first error, it is more relevant.  Fix all the tests
            that rely on the old behavior.
diff --git a/src/testdir/test_json.vim b/src/testdir/test_json.vim
index 86c47ae..c3b4382 100644
--- a/src/testdir/test_json.vim
+++ b/src/testdir/test_json.vim
@@ -181,7 +181,7 @@
   call assert_fails('call json_decode("{\"n\",1}")', "E491:")
   call assert_fails('call json_decode("{-}")', "E491:")
   if has('float')
-    call assert_fails('call json_decode("{3.14:1}")', "E474:")
+    call assert_fails('call json_decode("{3.14:1}")', "E806:")
   endif
 
   call assert_fails('call json_decode("[foobar]")', "E491:")
@@ -292,25 +292,25 @@
   call assert_equal({'n': 1}, js_decode('{"n":1,}'))
   call assert_equal({'n': '1'}, js_decode("{'n':'1',}"))
 
-  call assert_fails('call js_decode("\"")', "E474:")
-  call assert_fails('call js_decode("blah")', "E474:")
-  call assert_fails('call js_decode("true blah")', "E474:")
-  call assert_fails('call js_decode("<foobar>")', "E474:")
+  call assert_fails('call js_decode("\"")', "E491:")
+  call assert_fails('call js_decode("blah")', "E491:")
+  call assert_fails('call js_decode("true blah")', "E488:")
+  call assert_fails('call js_decode("<foobar>")', "E491:")
 
-  call assert_fails('call js_decode("{")', "E474:")
-  call assert_fails('call js_decode("{foobar}")', "E474:")
-  call assert_fails('call js_decode("{\"n\",")', "E474:")
-  call assert_fails('call js_decode("{\"n\":")', "E474:")
-  call assert_fails('call js_decode("{\"n\":1")', "E474:")
-  call assert_fails('call js_decode("{\"n\":1,")', "E474:")
-  call assert_fails('call js_decode("{\"n\",1}")', "E474:")
-  call assert_fails('call js_decode("{-}")', "E474:")
+  call assert_fails('call js_decode("{")', "E491:")
+  call assert_fails('call js_decode("{foobar}")', "E491:")
+  call assert_fails('call js_decode("{\"n\",")', "E491:")
+  call assert_fails('call js_decode("{\"n\":")', "E491:")
+  call assert_fails('call js_decode("{\"n\":1")', "E491:")
+  call assert_fails('call js_decode("{\"n\":1,")', "E491:")
+  call assert_fails('call js_decode("{\"n\",1}")', "E491:")
+  call assert_fails('call js_decode("{-}")', "E491:")
 
-  call assert_fails('call js_decode("[foobar]")', "E474:")
-  call assert_fails('call js_decode("[")', "E474:")
-  call assert_fails('call js_decode("[1")', "E474:")
-  call assert_fails('call js_decode("[1,")', "E474:")
-  call assert_fails('call js_decode("[1 2]")', "E474:")
+  call assert_fails('call js_decode("[foobar]")', "E491:")
+  call assert_fails('call js_decode("[")', "E491:")
+  call assert_fails('call js_decode("[1")', "E491:")
+  call assert_fails('call js_decode("[1,")', "E491:")
+  call assert_fails('call js_decode("[1 2]")', "E491:")
 
   call assert_equal(s:varl5, js_decode(s:jsl5))
 endfunc