Handle errors

[jtpio]

At the moment we wrap the debug requests in try catch blocks, and log the errors to the console:

https://github.com/jupyterlab/debugger/blob/80fb3d738c3405a3c65303d4015da2148925d810/src/service.ts#L109-L118

Instead we should handle the error properly, for example by showing an error message to the user.