Adding new function that allows us to stop using the @ error suppression operator...

Adding new function that allows us to stop using the @ error suppression operator but still keep notices and errors off screen.  Includes two sample uses: file uploads on the compose screen and session_start(), although the latter is very noisy in both the logs and in DEVEL's on-screen developer error handler display.  It might be best to add a flag to turn these errors off in the logs too.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@12763 7612ce4b-ef26-0410-bec9-ea0150e637f0