python3: KeyError to use format

Changes KeyError signatures to use `format` instead of `%`

partial: https://github.com/vmware/pyvmomi/issues/55
Malformed Faults fail in non-informative ways

* Malformed Faults: introduces test for fault handler
* introduces ParserError which includes the malformed XML
* testing demonstrates ParserError includes malformed XML

closes: https://github.com/vmware/pyvmomi/issues/72