plots: fail silently on errors

[dberenbaum]

Bug Report

Description

dvc plots show has started failing silently when there are errors instead of reporting an error was found. It looks like this was introduced in 43195a8 (cc @skshetry).

Reproduce

Clone https://github.com/iterative/vscode-dvc-demo and make these changes to dvc.yaml:

$ git diff
diff --git a/dvc.yaml b/dvc.yaml
index 3a4fa26..cd91913 100644
--- a/dvc.yaml
+++ b/dvc.yaml
@@ -2,8 +2,8 @@ plots:
   - Accuracy:
       x: step
       y:
-        training/plots/metrics/train/acc.tsv: acc
-        training/plots/metrics/test/acc.tsv: acc
+        training/plots/metrics/train/acc.tsv: train/acc
+        training/plots/metrics/test/acc.tsv: test/acc
       y_label: accuracy
   - Loss:
       x: step

Then run dvc plots show.

Expected

Prior to this change, dvc plots show would return an error message like this:

$ dvc plots show
ERROR: Could not find provided field ('train/acc') in data fields ('step, acc').

[skshetry]

I think I did it in this way to fix #9025.
Before we just used to fail, but now we reuse the same merged spec over multiple revisions data.

So, you may see multiple messages in this case if we logged them:

$ dvc plots diff workspace $(git log --oneline --pretty=format:"%h" -n2)
WARNING: Could not find provided field ('acc/loss') in data fields ('step, loss').                                                                                                                                   
WARNING: Could not find provided field ('acc/loss') in data fields ('step, loss').
WARNING: Could not find provided field ('acc/loss') in data fields ('step, loss').

[dberenbaum]

Makes sense for diff but what about for show?