cache.c: do not ignore errors from print_slot()

If print_slot() fails, the client will be served an inferior response.
This patch makes sure that such an error will be returned to main(), which
in turn will try to inform about the error in the response itself.

The error is also printed to the cache_log, i.e. stderr, which will make
the error message appear in error_log (atleast when httpd==apache).

Noticed-by: Jim Meyering <jim@meyering.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.c

@@ -380,7 +380,7 @@ int main(int argc, const char **argv)
 	err = cache_process(ctx.cfg.cache_size, ctx.cfg.cache_root,
 			    ctx.qry.raw, ttl, process_request, &ctx);
 	if (err)
-		cache_log("[cgit] error %d - %s\n",
-			  err, strerror(err));
+		cgit_print_error(fmt("Error processing page: %s (%d)",
+				     strerror(err), err));
 	return err;
 }