Provided default case for setting a float to non-numeric values
Removed compiler conditions around some default cases

The literal number parser doesn't really need to get a warning about
not matching other strings. We're never going to match every possible
string because we only parse for a handful of characters.
Added ScriptError.UnknownError case

Only using it currently for throwing an exception in the literal parser
The default case for checking which type of literal was parsed should
never match, so it's safe to assume something's really wrong if we
actually encounter that.