Catch Errors in SafeRequestHandler

The current behavior if an `Error` is thrown in the Selendroid server:
- Netty handles the error and Selendroid standalone sees a generic `NoHttpResponseException` in [`ProxyToDeviceRequestHandler.handleRequest`](https://github.com/selendroid/selendroid/blob/master/selendroid-standalone/src/main/java/io/selendroid/standalone/server/handler/ProxyToDeviceHandler.java).

The new behavior:
- Selendroid standalone sees the error message and stack trace.

In both cases the Selendroid server handles the error and tries to continue. The Android OS can still decide to kill the process if needed (e.g. in case of an OOM error).