Don't throw an exception when php-xml is missing

[nickvergessen]

@LukasReschke I guess we can also backport this later, so up to you if 11 or not

Fix #2180

[mention-bot]

@nickvergessen, thanks for your PR! By analyzing the history of the files in this pull request, we identified @ringmaster, @LukasReschke and @bartv2 to be potential reviewers.

[LukasReschke]

I guess this is because the compatibility check is performed later? Would an alternative be to move the compatibility check higher?

[nickvergessen]

Well the check uses language, therefor requires the user session, therefor session apps need to be loaded already => 🙈

[nickvergessen]

On the otherhand this stuff doesnt happen accidentially. So I guess an english only message would also be fine.

[rullzer]

I would be fine by not having this message translated.

[MorrisJobke]

@nickvergessen Could you fix this?

[MorrisJobke]

@nickvergessen Could you fix this?

Why do we even initialize a session if the instance is not yet installed. Maybe we could fix this the other way around.

[nickvergessen]

Sorry that will be way more work. Feel free to do that. It needs major changes in base.php and the Setup class.