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