plots: document no-error policy

[pared]

In treeverse/dvc#5984 we stopped raising exceptions from show/diff for plots/metrics/params/experiments
Seems that in some cases it might be confusing:
https://discord.com/channels/485586884165107732/485596304961962003/877829757834784768

We should mention that lack of some results is possible if we have problems acquiring the data.

[jorgeorpinel]

Thanks @pared . I assigned you but please lmk if that doesn't work.

[jorgeorpinel]

Haven't heard from this confusion again.