Story: handle gracefully lost internet connection

[shcheklein]

Extension version: v0.2.12
Demo project: https://github.com/iterative/vscode-dvc/tree/dd048e382be8334e438467e9a161ec7eb7044837

It's running dvc plots in the loop, experiments table is shown in-complete (and no error messages), loaders are spinning in the loop.

After connection is restored there is not way to refresh the table w/o reloading the window to see DVC tracked metrics.

[mattseddon]

Relates to #1649

• Add a refresh button to the plots webviews (consider only missing image)
• Only try to fetch revision once