fix(linux): Gracefully handle keyboard download failure

In the past we already gracefully handled keyboard download failures
when the user triggered the download from the km-config UI. This
change deals with the command line side of things, most often when
the user clicks on the "Install keyboard" button on keyman.com. If
the download fails we already output an error message and now don't
try to install the non-existing .kmp file.

Fixes #6284.