Include url in 403 error message (#153)

This makes debugging some situations a bit easier.

lib/client.js

@@ -622,7 +622,7 @@ class PlugAPI extends EventEmitter3 {
                             });
                         } else if (Object.is(err.statusCode, 403)) {
                             process.nextTick(() => {
-                                throw new Error(err);
+                                throw new Error(`${opts.url} responded with ${err.statusCode} (${err.statusMessage}`);
                             });
                         }