friendlier error messages for missing chunk managers

[keewis]

The current error message when trying to use a chunked-array related method without actually having a chunk manager available is:

unrecognized chunk manager dask - must be one of: []

That's pretty confusing, so this catches the case where no chunk manager is available and raises an error with guidance on how to fix that.

• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[max-sixty]

Much better!

[mathause]

Thanks - I had a PR on this but don't mind closing mine in favor of this one. There is also #7963 which seems related.

• closes missing chunkmanager: update error message #8803

[keewis]

wow, I don't know how I missed two open PRs that aim to do something similar in different ways. Which one do we take?

If we merge this one your PR might still be valuable since it also changes the error message if there are chunk managers but not the one that was requested.

[dcherian]

Shall we merge?

[TomNicholas]

wow, I don't know how I missed two open PRs that aim to do something similar in different ways. Which one do we take?

Sorry for dropping the ball on reviewing / merging these guys 😞

Let's merge this one.

If we merge this one your PR might still be valuable since it also changes the error message if there are chunk managers but not the one that was requested.

This change would also be useful but is much less likely to come up.

[TomNicholas]

While this could be further improved in cases like cubed being installed but dask not (when there would not be 0 chunkmanagers installed), this PR on it's own addresses the confusing error that 99% of users are encountering so should be merged asap.

[keewis]

we probably also want to prefer the more specific error over the generic n_chunkmanagers == 0 error (which would also allow passing a chunkmanager object if no chunk manager is available via the entrypoints)

[keewis]

@dcherian, @TomNicholas, this should be ready for a final review (and can hopefully still make it into the release looks like I was late by a couple minutes)

[dcherian]

we can always release more!

[keewis]

the failing zarr tests were because of the automatic chunking in open_zarr (chunks="auto")

[TomNicholas]

@keewis I made some very minor changes - feel free to merge this.

[keewis]

sorry for letting this stagnate for a month, I've been busy. I'll merge this tomorrow, should anyone be around and wish to review one final time.