#47 Add error-levels to phpCMS

[Thilo Wagner]

See http://phpcms.de/forum/index.en.html?topic=5358:

Classify the error-messages for example in three
categories: notices, warnings and critical-errors, and
then make it possible to configure via the admin GUI for
which error-levels phpCMS should output messages in
debug mode.
With debug mode switched off, notices and warnings
shouldn't produce any error while critical-errors should
cause the parser to load the defined error-page.

ciao... Iggi