OpenVPN: Clarify fundamental crypto errors but also warnings in WUI

OpenVPN: Clarify fundamental crypto errors but also warnings in WUI

Since OpenVPN-2.4.x, a lot of changes has been introduced. This patch should help the users for better understanding of errors in the cryptography.
It includes also potential warnings for upcoming changes and needed adjustments in the system.
This can also be extended in the future for upcoming configuration changes.

Signed-off-by: Erik Kapfer <erik.kapfer@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>