Fix sporadic NoMatches error reported from Header on app shutdown

[slawek-es]

During application shutdown, particularly when running via App.run_test(), it seems that the Header's watcher callback on title or sub_title is called while the Header's children, such as the HeaderTitle, are already unmounted. This results in a NoMatches error being raised.

We fix this by ignoring the NoMatches error, like we did with NoScreen in the recent "hardening" commit 604b04d.

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[mzebrak]

I tried to follow the issue of NoMatches raised in the Header widget because we have a similar workaround for this problem in our application and I feel responsible for writing this and questioning the changes proposed here.

I may be wrong, but I think this MR is more like a workaround and will hide more significant problems with --probably-- internal task handling?

Please see: #4268 (reply in thread) which reproduced this NoMatches error.

Later it resulted in #4305 which I tested and I was still able to reproduce NoMatches in the header including these changes.

But such a NoMatches try-catch will hide that error forever when there might be something else (bad) going on. In that case, most developers will probably need to also workaround similar issues with try-catch also, which is not so promising?