doc: make XMatch prepare tables error message state the two possible causes for failure

[ManonMarchand]

Fixes #1464

This PR modifies the error message to reflect the two possible causes for failure:

• either the table does not exist in the XMatch server (typo in the table name, or this VizieR table has no coordinates, and thus cannot be cross matched)
• or the user needs to provide colRa and colDec (the only info that was given before)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.68%. Comparing base (fbf55b0) to head (e447461).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3168      +/-   ##
==========================================
+ Coverage   67.49%   67.68%   +0.18%     
==========================================
  Files         232      229       -3     
  Lines       18446    18365      -81     
==========================================
- Hits        12450    12430      -20     
+ Misses       5996     5935      -61     

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[bsipocz]

Thank you so much!