Ambiguous message for non-plot target on plots modify

[pared]

DVC version: 1.8.4

After #4590 we support plotting for any "plot-able" file.

Support for plotting does not mean support for modifying the plot - since we don't have a place to write the information to.

plots modify-ing non plot file will result:
ERROR: Unable to find DVC-file with output '{filename}'
This message should probably be more clear in this context. Something like
ERROR: '{filename}' is not marked as plot in any DVC-file, and it cannot be modified.

Related to: treeverse/dvc.org#1809

[efiop]

Oops, my fault for marking everything in plots as being able to work without repo. Sending a quick fix.

[efiop]

Ah, ok, I misread the issue. This is not about working in non-dvc repo, but just dvc plots modify in general. Ok, nevermind.