Bubble up unknown enum variant error when parsing XML in the server (#1485)

XML deserialization of enums is currently broken.

This commit should have been done as part of #1398, but we only tested
that patch using the restJson1 protocol.

The converter from the unknown enum variant error into
`aws_smithy_json::deserialize::Error` has been removed from
`ServerEnumGenerator`, since it's protocol-specific logic. We instead
use `map_err` in the protocol-specific parsers before bubbling up using
`?`.

Fixes #1477.