Files should not be created when LESS et al. fail

[mythmon]

This is a general issue, but I'm going to use a specific case to demonstrate. In Kitsune, if LESS is not installed, or LESS_BIN is misconfigured and jingo-minify can't find LESS, than those style files are still generated. In particular, for every *.less, file, an empty *.less.css file is generated.

This is a problem because even when LESS is installed after this, those empty files cause jingo-minify to not regenerate that style file. This is frustrating and difficult to track down.

Ideally one or both of these would happen:

• If the command to compile a style file (*.less, for example) fails (return code != 0), thent he output file should not be created.
• If the output file exists, but is 0 bytes, the compilation command will be called anyways (currently jingo-minify doesn't regenerate the file unless the source has been changed.)