Stop referring to "UnknownError" as it is not part of DOM4 (closes #28)

Stop referring to "UnknownError" as it is not part of DOM4. Also use
DOMException instead of DOMError which is deprecated.
Merge pull request #56 from cdumez/UnknownError_removal

Stop referring to "UnknownError" as it is not part of DOM4 (closes #28)