Downgrade Cask install errors to warnings

Including the case where a Cask is already installed.
Always continue installing when multiple Casks are specified,
only raising an exception at the end of the command (if some
portion of the attempted install actions failed).  Never
exit with an error code if "already installed" was the only
problem seen during the run.

Also tweak error messages.

Fixes #1347, #2677, #4785

Required disabling two tests regarding suggestions on Cask
spelling errors.