Add useful error messages about missing password/token file in login

[jacalata]

Related to https://github.com/tableau/tabcmd/issues/168

Current behavior: python -m tabcmd login --server https://online.tableau.com --site tabcmd --token-name jac --token-file nonexistent.txt -- output says there was an error signing in

Expected behavior: output says the token file (or password file) was not found