Issue #5067: improve some json error messages.
Patch by Serhiy Storchaka.
diff --git a/Modules/_json.c b/Modules/_json.c
index 0924873..2a71b9f 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -634,7 +634,7 @@
 
             /* read key */
             if (str[idx] != '"') {
-                raise_errmsg("Expecting property name", pystr, idx);
+                raise_errmsg("Expecting property name enclosed in double quotes", pystr, idx);
                 goto bail;
             }
             key = scanstring_unicode(pystr, idx + 1, strict, &next_idx);
@@ -655,7 +655,7 @@
             /* skip whitespace between key and : delimiter, read :, skip whitespace */
             while (idx <= end_idx && IS_WHITESPACE(str[idx])) idx++;
             if (idx > end_idx || str[idx] != ':') {
-                raise_errmsg("Expecting : delimiter", pystr, idx);
+                raise_errmsg("Expecting ':' delimiter", pystr, idx);
                 goto bail;
             }
             idx++;
@@ -695,7 +695,7 @@
                 break;
             }
             else if (str[idx] != ',') {
-                raise_errmsg("Expecting , delimiter", pystr, idx);
+                raise_errmsg("Expecting ',' delimiter", pystr, idx);
                 goto bail;
             }
             idx++;
@@ -777,7 +777,7 @@
                 break;
             }
             else if (str[idx] != ',') {
-                raise_errmsg("Expecting , delimiter", pystr, idx);
+                raise_errmsg("Expecting ',' delimiter", pystr, idx);
                 goto bail;
             }
             idx++;