ui-patch: make sure to send http headers

Requesting a text/plain patch with bad commit id made cgit send text
without proper http headers. This results in "500 Internal Server Error"
with "Premature end of script headers" in server logs.
So print http headers before error message and return.

Signed-off-by: Christian Hesse <mail@eworm.de>
Reviewed-by: John Keeping <john@keeping.me.uk>