Fix / test mpc error messages 2022.07

[mkelley]

Update the MPC.get_observations error message to direct the user to the Minor Planet Center in order to help determine if the error is due to a problem with astroquery or MPC.

Also, test MPC.get_ephemeris with an object that exists, but apparently has no orbital elements in the database.

Addresses #2464

[pep8speaks]

Hello @mkelley! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers:

Comment last updated at 2022-07-13 15:21:54 UTC

[codecov[bot]]

Codecov Report

Merging #2466 (1ec3b88) into main (7277870) will decrease coverage by 2.94%. The diff coverage is n/a.

:exclamation: Current head 1ec3b88 differs from pull request most recent head 6097b39. Consider uploading reports for the commit 6097b39 to get more accurate results

@@            Coverage Diff             @@
##             main    #2466      +/-   ##
==========================================
- Coverage   65.86%   62.92%   -2.94%     
==========================================
  Files         233      133     -100     
  Lines       17907    17302     -605     
==========================================
- Hits        11795    10888     -907     
- Misses       6112     6414     +302     

Impacted Files	Coverage Δ
astroquery/mpc/core.py	88.32% <ø> (-0.07%)	:arrow_down:

... and 180 files with indirect coverage changes

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[mkelley]

Thanks!