Check that --output specifies a file

[DamienCassou]

I tried running this:

./melpa-stable-packages.sh --melpa ../melpa/ --output /tmp/
[...]

after hours of work, I got this error message:

melpa2nix: /tmp/: openBinaryFile: inappropriate type (Is a directory)

could it be possible to say that right from the beginning? This would have hours of computation :-).

Also, what about accepting a directory as input and writing a file with a standard name in it?