Render full shape IDs in server error responses (#1982)

RestJson1 implementations can denote errors in responses in several
ways. New server-side protocol implementations MUST use a header field
named `X-Amzn-Errortype`.

Note that the spec says that implementations SHOULD strip the error
shape ID's namespace. However, our server implementation renders the
full shape ID (including namespace), since some existing clients rely on
it to deserialize the error shape and fail if only the shape name is
present. This is compliant with the spec, see
https://github.com/awslabs/smithy/pull/1493.

See https://github.com/awslabs/smithy/issues/1494 too.