MXS-929: Add errors to the modulecmd system

The modules can now return human-readable error messages to the caller of
the function. The internals of the modulecmd system also use this to
return errors to the users.

The error messages can be retrieved with a common error function which
should make it easy to use in various clients.

server/core/config.c

@@ -2010,7 +2010,7 @@ config_truth_value(char *str)
     {
         return 0;
     }
-    MXS_ERROR("Not a boolean value: %s", str);
+
     return -1;
 }