Add file type checking to planex-fetch

[euanh]

planex-downloader checked whether the result of a download looked like an archive, and retried the download if it didn't. planex-fetch should do the same thing.

There is a Python library to wrap around libmagic, but it is more or less as ugly as just shelling out to file, as downloader used to do.

[simonjbeaumont]

Removing bug label now fetch has been fixed to not blat the file in the failure case. The enhancement suggestion remains valid.