Return different exit codes to distinguish errors

[dpordomingo]

caused by #162::comment.r307650453

[...] it may be nice to also return different return codes instead of using 1 for all of them.

This could be considered with #165

Originally posted by @se7entyse7en

[dpordomingo]

That would be nice.
imo we should have a design doc to consider which exit codes do we want, and maybe to regularize error message.