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

tests/test_util_file.test

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+# Common definitions
+if test -z "$srcdir"; then
+    srcdir="${0%/*}"
+    test "$srcdir" = "$0" && srcdir=.
+    test -z "$srcdir" && srcdir=.
+fi
+. "$srcdir/test-defs.sh"
+
+run_output_test test_util_file "$srcdir"
+_err=$?
+
+exit $_err