fix(server): Report original error message (#3415)

Display original error message of plugins like: karma-browserstack-launcher, karma-cbt-launcher.

Fixes #3414

Author: Borewit

lib/server.js

@@ -395,12 +395,12 @@ class Server extends KarmaEventEmitter {
     }
 
     processWrapper.on('unhandledRejection', (error) => {
-      this.log.error('UnhandledRejection')
+      this.log.error(`UnhandledRejection: ${error.message || String(error)}`)
       reportError(error)
     })
 
     processWrapper.on('uncaughtException', (error) => {
-      this.log.error('UncaughtException')
+      this.log.error(`UncaughtException:: ${error.message || String(error)}`)
       reportError(error)
     })
   }