Don't destroy existing outputPath on fail

Compile writes out to os.tmpdir, and if successful, it then moves the directory to the user defined output path and cleans up after itself. This allows for a compile to fail, but won't blow away the existing directory.

I was using Harp for our dev and was thrown off by it destroying the contents of www when it errored. So I fixed that. Hopefully useful to others and PR-mergable.