Improve invalid branch name error message
    
    Currently, if the resolved release name does not match any supported
    pattern ('rawhide', 'f38' or so), the following error is printed:
    
        (foo) $ fedpkg prep
        Could not execute prep: Could not find the release/dist from branch name foo
        Please specify with --release
    
    This is fine when the current Git branch name was used when resolving
    the release. However, the exact same error is printed even if the
    '--release' option was used, like this:
    
        $ fedpkg --release foo prep
        Could not execute prep: Could not find the release/dist from branch name foo
        Please specify with --release
    
    The error message is split into two cases depending on if --release was
    used (detected by checking if 'self.dist' is truthy):
    
        (foo) $ fedpkg prep
        Could not execute prep: Could not find release from branch name 'foo'. Please specify with --release.
        $ fedpkg --release foo prep
        Could not execute prep: Invalid release 'foo'.
    
    JIRA: RHELCMP-11465
    Fixes: https://pagure.io/rpkg/issue/671
    Merges: https://pagure.io/fedpkg/pull-request/518
    
    Signed-off-by: Otto Liljalaakso <otto.liljalaakso@iki.fi>