Bump version

Signed-off-by: Travis Rhoden <trhoden@redhat.com>
1.5.20
return non-zero when gatherkeys fails

Immediately abort and exit when the gatherkeys command fails.

Define a new exception, KeyNotFoundError, and raise that from
fetch_file() when the requested file does not exist.  Since we
now raise an exception on error, there was no longer any need to
return a value, since the previous return was just a True/False of
whether the file was found.

Signed-off-by: Travis Rhoden <trhoden@redhat.com>