Distinct error if all mutants are unviable

[sourcefrog]

If all the tested mutants are unviable then most likely something is wrong with building in the scratch directory. This seems to be happening for example in https://github.com/sourcefrog/cargo-mutants/actions/runs/10494305908/job/29070538285.

This is more of a problem with --baseline=skip where we won't get the chance to discover it during the baseline test.

In some cases this could be a false positive: suppose a highly sharded runner gets just one or two tests, and it happens that they're both unviable even when other shards succeed. (This could eventually be handled better with a tool that recombines the shard output.)