Backport ErrorStatus fix

[mkalcok]

Add ErrorStatus so that getting Unit/App.status when the status is "error" does not raise.

This is merely a backport of #875 (that fixes #874) into the 1.5-maintenance branch.

Checklist

• Have any types changed? If so, have the type annotations been updated?
• If this code exposes model data, does it do so thoughtfully, in a way that aids understanding?
• Do error messages, if any, present charm authors or operators with enough information to solve the problem?

QA steps

added tests.

Bug reference

Backports fix for #874 into the 1.5-maintenance branch.

Changelog

• Fixes bug Error status causes update-status hook to fail #874, where charms whose unit or app is in error status can't access their own status.

[mkalcok]

CI failures do not seem to be related to the code changes. Fix for the CI proposed in #928

[benhoyt]

This looks fine, thanks for that. Would you mind bringing your branch up to date now that #928 is merged? And then I can merge this and release 1.5.5.

[mkalcok]

Thanks for the review. I rebased the PR branch.

[benhoyt]

Seems okay to me to backport this fix to the 1.5 branch -- thanks.

[benhoyt]

@mkalcok This is now released in ops version 1.5.5, which you can install with pip install ops==1.5.5.

[mkalcok]

Awesome, thank you for fast release :)