plots diff: return revision level error instead of failing when revision not available

[mattseddon]

The issue was originally reported in iterative/vscode-dvc#4333

If plots diff is called with a revision that appears in the experiments table but is not available (e.g. experiment failed) then the entire command fails.

From @dberenbaum

The only fix I can see in the CLI would be to still show the other revisions, but I still don't like that UX since I would be wondering where the motor-abac plots are.

In the extension, we have the ability to show that a single revision failed to return/errored out. It would be good to adjust the output of --json to return this error instead of failing completely. The non---json behaviour could remain the same.

[dberenbaum]

The failed revision is shown in the output. Can VS Code parse this and drop it from the dvc plots diff call? Or check if the revision exists before passing it to dvc plots diff? I would prefer not to have different behavior for --json if there is another option.

[mattseddon]

The failed revision is shown in the output. Can VS Code parse this and drop it from the dvc plots diff call?

This involves (probably regex) parsing of a free text error that we've avoided for obvious reasons up until this point.

Or check if the revision exists before passing it to dvc plots diff?

The revision that we are looking for do exist in the exp show data and do not relate to queued experiments that have not yet started.

I would prefer not to have different behavior for --json if there is another option.

👍🏻. This might not be the way to go. We can discuss potential solutions and whether or not this is even an issue in the linked ticket.