Send all invalid mentions output to stderr

[zdrve]

To ensure the "validate mentions" diagnostics come out in an understandable sequence.

It currently (example) says:

It generally means that you're trying to add a notification to somewhere for the first time
... (many lines later) ...
Invalid mentions found

which makes no sense. So change it to:

Invalid mentions found
...
It generally means that you're trying to add a notification to somewhere for the first time

[grosser]

✨

[grosser]

v1.148.1