missing chunkmanager: update error message

[mathause]

When dask is missing we get the following error message:

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

this could be confusing - the error message seems geared towards a typo in the requested manager. However, I think it's much more likely that a chunk manager is just not installed. I tried to update the error message - happy to get feedback.

[TomNicholas]

I swear to god we already had an issue / PR about exactly this somewhere (I think with @dcherian ?), but now I can't find it.

Regardless, I think your change does clarify the error.

[kmuehlbauer]

https://github.com/pydata/xarray/pull/8736 this? @TomNicholas

[mathause]

Or #7963?

[mathause]

I would like to merge this but after checking #7963 and #8301 I am a bit confused.

• Should I update the error message to accommodate for the "second chunk manager" (#8301 (comment))?
• If so what is the "second chunk manager"?
• Here I update the error message in guess_chunkmanager while #7963 updates the error message in get_chunked_array_type. Without knowing all the details I think it's more likely to run into the error in guess_chunkmanager (because to trigger the error in get_chunked_array_type there already needs to be a 'chunked array').