Fix a potential match error when show-ing failed tasks

[lefou]

I've witnessed this issue and also, that this PR fixes it, but I couldn't find a way to reproduce it in current test infrastructure. But better have the fix now then much later, I guess.

The situation is with Mill 0.10.1 and the project described in #1028 (comment)

$ rm -r ~/.cache/coursier/v1/https/repo1.maven.org/maven2/org/slf4j/slf4j-api/1.7.25/ ; mill clean ; mill -j 0 show __.resolvedIvyDeps
No mill version specified.
You should provide a version via '.mill-version' file or --mill-version option.
Using mill version 0.10.1
[1/1] clean 
No mill version specified.
You should provide a version via '.mill-version' file or --mill-version option.
Using mill version 0.10.1
[#00] [1/1] show 
[#05] [1/1] show > [97/120] foo[2].resolvedIvyDeps | Downloading [2/2] artifacts (~40/3873 bytes)
1 targets failed
show scala.MatchError: Watched(None,List()) (of class mill.util.Watched)
    mill.main.MainModule.$anonfun$show$5(MainModule.scala:275)
    mill.api.Result$Failure.$anonfun$map$1(Result.scala:67)
    scala.Option.map(Option.scala:242)
    mill.api.Result$Failure.map(Result.scala:67)
    mill.api.Result$Failure.map(Result.scala:66)
    mill.main.MainModule.$anonfun$show$1(MainModule.scala:274)
    mill.define.Task$TraverseCtx.evaluate(Task.scala:379)