ENH: improving error messaging for invalid SIAv2 inputs

[bsipocz]

This keeps bugging me, so while we think about possible workarounds in #692, I would just make the error message more descriptive than the current one propagated from astropy:

TypeError: Scalar 'SkyCoord' object has no len()

Technically this is an API change as we change the error type from TypeError to ValueError but I feel this is still backportable improvement (no user code should have try/excepted on this TypeError).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.02%. Comparing base (30a4ecb) to head (6469682).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #693      +/-   ##
==========================================
+ Coverage   84.00%   84.02%   +0.01%     
==========================================
  Files          80       80              
  Lines        8522     8524       +2     
==========================================
+ Hits         7159     7162       +3     
+ Misses       1363     1362       -1     

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[msdemlei]

I trust this is a UI improvement and approve of it as such. However, even a quick glance at the SkyCoord docs in astropy did not reveal the logic of pos.size<4 to me (it doesn't help that my version of the astropy docs doesn't even mention the size attribute). I also couldn't locate the place where an appropriately-sized SkyCoord would turn into a polygon (if that's what it is). So: I'd say a comment is in order here. But perhaps I'm just being particularly dense here.

While you're editing this: Could you add the missing "been" in _validate_pos' docstring?

Feel free to merge, though.

[bsipocz]

Yes, the size is inherited property and those seem to be not exposed to the API docs, I found it with TAB completion (and now its only mention in the docs is here: https://docs.astropy.org/en/stable/coordinates/skycoord.html#attributes).

[bsipocz]

Feel free to merge, though.

I'll go ahead with this, with the known caveat that this may only land in 1.7.1 as I plan to follow-up on our discussion in #692 and add a default small radius for v1.8