Add format checks for printf-style functions

Added format checks to functions that expect printf style arguments and
fixed any broken calls to these functions.

include/maxscale/modulecmd.h

@@ -256,7 +256,7 @@ bool modulecmd_call_command(const MODULECMD *cmd, const MODULECMD_ARG *args, jso
  * @param format Format string
  * @param ... Format string arguments
  */
-void modulecmd_set_error(const char *format, ...);
+void modulecmd_set_error(const char *format, ...) mxs_attribute((format (printf, 1, 2)));
 
 /**
  * @brief Get the latest error generated by the modulecmd system