Show warning when invalid user was passed

[PVince81]

Sometimes there are bugs that cause setupFS() to be called for
non-existing users. Instead of failing hard and breaking the instance,
this fix simply logs a warning.

This is mostly a protection against other bugs. This code path was already triggered by three different bugs (from which some were fixed already).

Please review @icewind1991 @schiesbn @Xenopathic

[icewind1991]

👍 makes sense

[RobinMcCorkell]

LGTM 👍

[PVince81]

Tests failing on master, let's wait for them to be fixed.

[LukasReschke]

That test will be fixed when #12085 is discussed and merged...

[LukasReschke]

... let skip the test for now ... - creating PR.

[scrutinizer-notifier]

The inspection completed: 1 updated code elements

[LukasReschke]

@owncloud-bot Retest this please.

[PVince81]

Jenkins and force push... I'll submit a separate PR

[PVince81]

Superseded by #12290

[PVince81]

@karlitschek @jnfrmarks I'd like to get this backported.

This issue has been reported at least 4 times already, with at least three different causes / code paths:

1. recovery key issue: fixed by [stable7] Fix file upload to ext storage when recovery key is enabled #12131
2. LDAP old/disabled user
3. Possibly another LDAP issue

Even though the root cause might not be critical in itself, the code path it follows leads to this exception, which itself leads to feature breakage. The fix mitigates that problem by logging a warning instead of throwing and exception, making OC work again.

[karlitschek]

Please backport. Thanks

[PVince81]

stable7: f64c6c9

@jnfrmarks reproduction steps are same like #12131