Show valid instance names when given a dandi:// URL with an unknown instance

[jwodder]

Closes #809.

[codecov]

Codecov Report

Merging #815 (aab3af5) into master (bc93b64) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #815      +/-   ##
==========================================
+ Coverage   84.56%   84.63%   +0.06%     
==========================================
  Files          61       61              
  Lines        6161     6168       +7     
==========================================
+ Hits         5210     5220      +10     
+ Misses        951      948       -3     

Flag	Coverage Δ
unittests	84.63% <100.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/dandiarchive.py	85.35% <100.00%> (+1.76%)	⬆️
dandi/tests/test_dandiarchive.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bc93b64...aab3af5. Read the comment docs.

[yarikoptic]

Looks great to me, thank you @jwodder . In the future (if happens) we might relax the test a bit to not list all possible instances in the expected string, or code that list up since it would be annoying to dig out / adjust that test every time we need to add or remove an instance