Issue #189: Eliminate use of MC_ERROR from json_util.c, and add a json_util_get_last_err() function to retrieve the error for those callers that care about it.

Add tests and descriptions for the functions in json_util.c

.gitignore

@@ -47,6 +47,7 @@
 /tests/test_printbuf
 /tests/test_set_serializer
 /tests/test_compare
+/tests/test_util_file
 /tests/*.vg.out
 /tests/*.log
 /tests/*.trs