Export missing function - couch_httpd:send_error/2

Hello All!
This function is used by couch_plugins_httpd.erl but unfortunately was not properly exported. So let's export it.

See:

* https://github.com/apache/couchdb/blob/master/src/couch_plugins/src/couch_plugins_httpd.erl#L36
* https://github.com/apache/couchdb/blob/master/src/couchdb/couch_httpd.erl#L30