[BUGFIX] Prevent Exception when form folder doesn't exists

[BUGFIX] Prevent Exception when form folder doesn't exists

When for ext:form an addition allowedFileMounts is set to an
not existing folder you get an FolderDoesNotExistException when
opening the form editor.

This patch catches the FolderDoesNotExistException and ignores the
folder when not present.

Releases: master
Resolves: #80670
Change-Id: I40994cf0f7b7a005c0b902736c71dcb006b612dd
Reviewed-on: https://review.typo3.org/52352
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>