fix gotocomputer when key_filename missing

[ltalirz]

fix #3588
fix #3401

• the "verdi calcjob gotocomputer" command was broken when used on computers that were configured without a value for the ssh key_filename.
• "verdi calcjob gotocomputer" obviously does not work when no authinfo is
  present for the computer to go to. There was a bug in printing the exception, resulting in an exception bubbling up to user space (instead of a critical error with a helpful message).

[ltalirz]

I noticed that there does not seem to be any test of the gotocomputer command.
Perhaps not that easy to test but I guess also not impossible. @giovannipizzi Perhaps you could open an issue with some pointers on where this test should go / existing tests one could borrow code from.

[giovannipizzi]

It would be great to have a test for this, but I'm not sure how... One should have a configured computer and a calculation run in there (one could use the test_daemon.py file that actually runs simulations, but I'm not sure this is over SSH - actually maybe @sphuber is changing to local in #3571?). But then one needs to test if an interactive shell is opened... Not sure how to do it exactly.

Maybe one could instead just get internally the command that is generated ssh ... and append something execute (e.g. create an folder with a given file in /tmp/XXX, then append ls /tmp/XXX to the ssh command, and check if the result shows correctly the folder content? Even if I'm not sure how robust this test would be).