Add "show if available" variant to be used in autocmds

[dbarnett]

We should document an autocmd a user can define to show coverage on certain paths as soon as they're opened, without seeing an error or warning about missing coverage (particularly in the async case). Currently there's no obvious way to get that behavior reliably.

The coverage plugin will need to document some clear contract for what .GetCoverage should throw/return and what .GetCoverageAsync should do to signal coverage was not found, and use that to surface error information to the user consistently.