The existing error pages are very basic and don't allow for
multi-lingual support or for conformity with other pages in a web site.
The aim of the callback functionality is to allow custom error pages to
be supported for calls to evhttp_send_error() by both calling
applications and Libevent itself.
A backward-incompatible change has been made to the title of error pages
sent by evhttp_send_error(). The original version of the function used
the reason argument as part of the title. That might have unforeseen
side-effects if it contains HTML tags. Therefore the title has been
changed to always use the standard status text.
An example of the error callback can be found in this
[version](https://github.com/libevent/libevent/files/123607/http-server.zip)
of the 'http-server' sample. It will output error pages with very bright
backgrounds, the error code using a very large font size and the reason.
Closes: #323 (cherr-picked from PR)
02905413fd