Prevent crash when cache shrink fails

[jgosmann]

Motivation and context:
This prevents from Nengo crashing (exiting with an exception) when the cache shrink fails because of an unavailable lock.

Fixes #1335.

Interactions with other PRs:
none

How has this been tested?

Added a test. It needs to access some cache internals which might not be the best style, but it seems to be the easiest way to test it and I don't think the index or lock should be public.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Where should a reviewer start?

Types of changes:

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the CONTRIBUTING.rst document.
• [n/a] I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jgosmann]

@tbekolay, it would be nice to have this in the release.

[tbekolay]

Added a fixup with some minor wording changes. Will merge shortly unless someone stops me!