fix: clearer error handling around nested arrays

[tychoish]

Closes #3120

[talagluck]

The current error returned is:

Error: Error during planning: Unable to fetch table provider for 'posts': Unsupported json type: cannot unwind nested arrays: must return objects from jaq expressions

Could we remove one of the "Error"s?

Also, I wonder if it's worth adding an example in the error message here (like "must return objects from jaq expressions, e.g. '.elements[]' instead of '.elements'"). This feels very clumsy, but this set of operations is something that I'm not very familiar with, and I imagine most people working with GlareDB here will have more experience with tabular data and trying to join in a json object instead of the other way around. It would be good to help people get unstuck, and when I search for the current error message, I don't get anything immediately helpful.

[tychoish]

Error during planning is a datafusion error, so I can't get rid of that error, and the outer error, I think is happening at the top level in the python bindings, so practically speaking there's not a good way to get out of that one.

[tychoish]

I think we could add a redirection to the docs or a perm-link to the docs, but I think in general, it's hard to write something that's clear and actionable in an error message in a way that's more helpful to the user.

We should definitely include this in the docs, however, because I think you're right that it is confusing.

Do you have any objections to merging this one?