modify --watch option: continue when SyntaxError

i want 'mocha --watch' continue when SytaxError.
becase rerun is a little difficultiy.
but was stop 'mocha --watch' when SytaxError.

this modify ...
 * when command start and SytaxError then stopped.
 * when 'mocha --watch' runninng and SytaxError then running.
change "console.error(e.stack)"
Merge pull request #944 from haru01/master

modify --watch option: continue when SyntaxError