Add handleServerStderr method + default logging

[alexheretic]

As discussed a little in #157 this PR adds stderr handling to a new public method:

/**
 * Called on language server stderr output.
 * @param stderr a chunk of stderr from a language server instance
 */
handleServerStderr(stderr: string, projectPath: string) {
  stderr.split('\n').filter(l => l).forEach(line => this.logger.warn(`stderr ${line}`));
}

• Restored stderr / childProcess error handling code.
• Sends stderr chunks to new method, default impl warns per line

Resolves #157

[damieng]

I think we still need this to report when starting a language server fails to the user.

[alexheretic]

How does this work though, node processes don't seem to have an error event as the previous (unused) code used to call this method.

[damieng]

https://nodejs.org/api/child_process.html#child_process_event_error

[alexheretic]

Ah right thanks, I was looking at process rather than child-

[damieng]

I'm okay with us logging by default. I wonder if we should chose .error ?

[alexheretic]

It was warn in the unused code previously. I don't mind!

[alexheretic]

This should be for each line too, the console handles it much better.

[alexheretic]

warn instead of error makes sense as stderr could be used for all sorts really.

[alexheretic]

@damieng I've squashed the change down and I think we've addressed all the issues. Are there any further problems do you think?