snapshot: don't reimplement cgit_print_error_page()

cgit_print_error_page() has the advantage that it sets a suitable HTTP
status code for the response.  Note that setting "mimetype" is redundant
here since it cannot have changed since being initialized in
cgit.c::prepare_context(), so we do not need to worry that
cgit_print_error_page() does not set it.

Signed-off-by: John Keeping <john@keeping.me.uk>