Improve error message when user tries `choosenim install`
I was trying to install a specific branch of the Nim repo, and kept trying the command `choosenim install "#branch_name"`. I should have been issuing the command `choosenim "#branch_name"` but this isn't well documented and there are no helpful error messages when the former is tried.

It'd be very nice if the help command better explained installing branches / commit hashes and if the error messages received after a user inputs  `choosenim install`, were improved.