Add MmgError exception raise when a mmg command fails

[ricardo0115]

Before this pull request exceptions raised by Mmg calls were completely silent, the idea is to raise an exception of type MmgErrorthat recalls the original causing exception which can be subprocess.CalledProcessError or FileNotFoundError

A minimal test case when no arguments are passed to mmg call is added in test_external.py

[kmarchais]

Thanks!