Error in preprocessor stops watch session.

When karma starts with `autoWatch: true` and a preprocessor fails after file modification (let's say coffee syntax error was introduced), whole runner is stopped.

More sane behaviour would be to emit error to the user, continue watching and try again after a file is modified.

Start with Karma's autoWatch flow and the preprocessor error path. Reproduce the issue by introducing a CoffeeScript syntax error after a file modification, then observe whether the runner stops. Done means the error is reported, the watch session continues, and a later file modification triggers another attempt.

Written by the indexing model from the issue text.