[QoL suggestion] Make adapter test discovery errors discoverable/survivable

[fnune]

Two things that should hopefully reduce the amount of "my adapter can't find tests" issues here, or at least improve their quality:

• When an adapter fails to discover tests for some reason (e.g. https://github.com/nvim-neotest/neotest/issues/348, https://github.com/nvim-neotest/neotest/issues/303), make other adapters in the same project continue running. Currently, if e.g. neotest-jest fails, neotest-python will not run at all and will report "no tests found".
• When an adapter fails, show an error in message or show something within :Neotest summary like an error sign.