fix: Log errors in the background subprocess

Currently there is not output if the background subprocess exits with an error,
for example because the configuration file has a syntax error. This commit fixes
that.
Merge pull request #116 from mzgol/error-handling

fix: Log errors in the background subprocess
fix: Pass node-specific command line params to the subprocess

This commit makes the karma task pass Node-specific parameters used to start
the Node process to its background subprocess. This is required e.g. to be able
to use the --harmony flag.

Similar pull request for Grunt itself (already merged):
https://github.com/gruntjs/grunt/pull/877