Note that this behavior is documented[0]:
"""
The XML data is parsed using the Expat parser from xml.parsers.expat – see its documentation for possible exceptions on ill-formed XML. Unknown elements will simply be ignored by the plist parser.
"""

Since this is documented and expat only has an exception type (xml.parsers.expat.ExpatError) I don't think is necessary to do anything here.
Unless you think that the doc should be rephrased, this can be closed again.

[0]: http://docs.python.org/library/plistlib.html#plistlib.readPlist