Error pages module

This improvement allows users to customize their http error pages, for a bunch of well known and predefined http error codes.

A separate attempt was also tried, to enable fully-customized http error handling by means of an httpfilter. This has proven to be pretty-faces incompatible (pretty sets the response committed, preventing from doing any forward/redirect/content modification on it).

For (sofar unforeseen) future use, errorfilter attempt has been left as a separate module, not included in the giavacms pom.xml official module list.