remove field from errors

we have standardized on indexOf in error.message by now

src/docker.js

@@ -58,7 +58,7 @@ async function testRegistryConfig(config) {
     if (config.provider === 'noop') return;
 
     const [error] = await safe(gConnection.checkAuth(config)); // this returns a 500 even for auth errors
-    if (error) throw new BoxError(BoxError.BAD_FIELD, error, { field: 'serverAddress' });
+    if (error) throw new BoxError(BoxError.BAD_FIELD, `Invalid serverAddress: ${error.message}`);
 }
 
 function injectPrivateFields(newConfig, currentConfig) {