revise string matching for node v0.10.x

I noticed the windows wrapper is no longer catching errors correctly (fatal errors are silent) with node v0.10.x because `throw e;` is now `throw err;` (https://github.com/mapbox/tilemill-win-launcher/blob/b503a2591d2bb4e4f9c90b3dc781861ce02ed784/tilemill.cc#L86). This pull fixes to just look for `Error:` but avoids matching bones `Client Error`:

Needs testing on OS X (ideally by @incanus) before merging.