pkgset: Emit better error for missing modulemd file
    
    The exceptions from libmodulemd are not particularly helpful as they do
    not contain information about what file caused it.
    
       modulemd-yaml-error-quark: Failed to open file: Permission denied (0)
    
    This patch should add the path to the problematic file into the message.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>