Pull back for now on switch of PEAR error mode to exceptions; seems to trigger out...

Pull back for now on switch of PEAR error mode to exceptions; seems to trigger out exceptions at various times we don't want them.

For instance this was throwing an exception for DB_DataObject::staticGet when there's no match... definitely not what we want when all our code expects to get a nice null.
Example of this causing trouble: http://gitorious.org/statusnet/mainline/merge_requests/131

Revert "Don't attempt to retrieve the current user from the DB while processing a DB error"

This reverts commit 68347691b0c7fb3f81415abd7fcdc5aec85cc554.

Revert "Use PHP exceptions for PEAR error handling."

This reverts commit d8212977ce7f911d4f9bd6e55f94aea059a86782.