Introduce some (commented-out) tests for enumeration codegen.
Rewrite FindEnumStringIndex to distinguish 'value not found' and 'exception raised'.

This is in preparation for handling those cases differently.
Handle exceptions from FindEnumStringIndex correctly.

The previous code would return success from the JSNative with a pending
exception, potentially leading to assertion failures inside the JS engine
later.
Implement support for default values for enumeration arguments.
auto merge of #2154 : Ms2ger/servo/enumerated-codegen, r=jdm