Allow send_error to send different content types

[colinnewell]

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.

[veryrusty]

This looks potentially useful, say with a send_error_as keyword.

Note that send_as started out as a plugin; which helped us iron out the syntax. You may want to consider the same approach for this.

[xsawyerx]

You would need to refactor the logic in send_as so it isn't repeated.