Clearer help text for apps with multiple extensions

Relates to #522

src/commands/app/run.js

@@ -42,7 +42,7 @@ class Run extends BaseCommand {
     const runConfigs = this.getAppExtConfigs(flags)
     const entries = Object.entries(runConfigs)
     if (entries.length > 1) {
-      this.error('You can only run one implementation at the time, please filter with the \'-e\' flag.')
+      this.error('Your app implements multiple extensions. You can only run one at the time, please select which extension to run with the \'-e\' flag.')
     }
     const name = entries[0][0]
     const config = entries[0][1]