Allow send_error to send different content types

In applications where there are routes outputting json and html functions like send_as make that work without too much of a problem. send_error appears to have all the logic to deal with a serializer, but it could do with a way of asking for a different one so that errors come out in a consistent format with the rest of the request rather than what the global application is configured for.

It looks relatively simple to implement assuming a reasonable syntax can be agreed upon for the helper. I'd be willing to create a pull request but I figured it was best to check it wasn't going to get shot down before doing so.

Start by tracing the existing send_error and send_as helpers, focusing on how send_error selects and invokes serializers. Clarify the helper syntax and verify that errors can select a serializer matching the rest of the request rather than the global application setting.