Better error messages when options are misplaced or missing

[drewblessing]

Example:
gitlab snippet add file.txt -t title -n name

Looks ok, right? But it gives a big ugly stack trace. In this case it was simply missing the project ID or name:
gitlab snippet add 13 file.txt -t title -n name

We should give some nicer messages that say something went wrong and here's maybe what happened...