decode(): Raise a uu.Error if no out_file is given but the file

decode(): Raise a uu.Error if no out_file is given but the file
    specified in the uu header already exists.  No additional
    workaround is provided since out_file=pathname is a deprecated
    interface, so it is better to simply pass a file-like object into
    out_file anyway.  This closes SF bug #438083.

Use isinstance() tests instead of type comparisons.