rrdimage: Improve CGI & cosmetic changes

rrdimage: Improve CGI & cosmetic changes

Write graph error messages to the system log, to simplify
further inspection by the user.
Add additional parameter check to prevent a possible redirect loop
if the URL format is changed in the future.

Cosmetic: Use underlining instead of background color for highlighting

Fixes #10643

Signed-off-by: Leo-Andres Hofmann <hofmann@leo-andres.de>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>