Stop using RuntimeError
    
    RuntimeError is "unspecified error", and generally should be used only when we
    don't know what happened and don't have a better error class to use. For the
    cases where standard errors match, let's just use FileNotFoundError,
    FileExistsError, and shutil.SpecialFileError. An additional class
    FileIsModifiedError is defined, because none of the commonly used types match.
    
    Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>