Report warnings if not sources are found and sent to the user

[tgodzik]

Version(s)
Scala CLI 1.5.4, but it doesn't happen reliably.

Describe the bug
dependencySources request is met with an empty response

To Reproduce
Try to start metals on a simple scripts as described in scalameta/metals#6900

Expected behaviour
We should report a warning in that case or ideally also retry fetching sources. (if that is a Bloop/Scala CLI race we might need to add more info to that warning)