plots: more helpful error message on corrupted/invalid `dvc.yaml`

When dvc.yaml is either corrupted or fails validation, dvc plots show/diff output shows DVC failed to load some plots for following revisions: workspace. There are a couple issues with this:

• It makes it seem like the issue is with one or more of the plots, when it's really about the structure of dvc.yaml and may be completely unrelated to plots.
• It makes it seem like some plots from that revision might still be shown, which isn't true.
• It doesn't give any indication of how to fix it. Other commands like dvc repro give output about what failed like:

'./dvc.yaml' is invalid.

While scanning a simple key, in line 24, column 1
  24 dfdf

Could not find expected ':', in line 25, column 1
  25

For dvc plots show, can we show something similar?

For dvc plots diff, this might not make sense by default since there could be many revisions. However, it still seems like we could improve the message. For example, it could say something like Failed to read dvc.yaml for the following revisions (add -v to see errors): workspace.

Start at the dvc plots show and dvc plots diff command paths and inspect how corrupted or invalid dvc.yaml errors are converted into the current revision message. Compare their behavior with the dvc repro error output described in the issue. Done means both commands clearly identify dvc.yaml loading or validation failures and provide an appropriate message for show and diff.