Catch errors when starting the server

[raboof]

When starting the server, we wait indefinitely until the http port file appears, even when the child process has already exited with an error code.

When the server fails with an error code, it would be nice if we could show feedback of this.

[raboof]

Something like master...raboof:showErrorStartingEnsime ?

(would need to add polling, tests, and actually handling the failed promise - but I'd like to get feedback on the approach first)