Add a header on thumb.php saying which thumb server on error.

Add a header on thumb.php saying which thumb server on error.

Only added if $wgShowHostnames is on. Adding the host name as a
header makes it much easier when debugging with command line tools,
since one often throws away the response body. This still keeps
the host name in an html comment in the response body as well,
like it was previously.

Change-Id: I9d15bd5e2abb4e7a8d3fe2a17fc546a1c5f3b70f