Fix __repr__ for unknown coordinate systems

[dstansby]

Fixes #2452

[ghost]

Thanks for the pull request @dstansby! Everything looks great!

[ghost]

Thanks for the pull request @dstansby! Everything looks great!

[wtbarnes]

Out of curiosity, in what context do you have an unknown coordinate system?

[dstansby]

@Cadair's original example was when 'CUNIT1' was 'STUART1', so maybe he invented his own coordinate system? I'm not sure to be honest, but I feel like if one can make an object without an error then __repr__ shouldn't raise an error.

[sunpy-backport]

The backport to 1.0 failed:

Commits ["3064b35e5562edd3c182d63cd242539c5451f4c8","932fc06717948815678c1135021c580e8a43c528"] could not be cherry-picked on top of 1.0

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport 1.0
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 3064b35e5562edd3c182d63cd242539c5451f4c8 932fc06717948815678c1135021c580e8a43c528
# Create a new branch with these backported commits.
git checkout -b backport-3569-to-1.0
# Push it to GitHub.
git push --set-upstream origin backport-3569-to-1.0
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is 1.0 and the compare/head branch is backport-3569-to-1.0.