[BUGFIX] Improve method annotation and type hinting in FAL

Add a missing "@throws" annotation and a namespaced comment.

Change-Id: Ibc929a01dbfdef9cf8730a8da259570786b6118c
Resolves: #50447
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/22572
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn