TMDM-7631: "readable message not display on web ui but server console"

Merges exception handling code in WebSaver with XtentisWebPort (client wrapper dedicated to Web UI use).
Delete obsolete class WebSaver.
Renames WebSaverTest to XtentisWebPortTest (and adapt test to what it does actually test: exception processing).
(https://jira.talendforge.org/browse/TMDM-7615)