download: handle mis-formatted build NVR
download: handle mis-formatted build NVR

Prior to this change, if a user ommitted the package name in the
argument to "rhcephpkg download", then rhcephpkg would crash with an
error, because it could not parse the build Name-Version-Release.

Crashes:

  rhcephpkg download 10.0.0-1

Correct:

  rhcephpkg download ceph_10.0.0-1

Catch the error and return a helpful message to the user in this case.

red-hat-storage/rhcephpkg,red-hat-storage/rhcephpkg