Surface failed checks' underlying causes in fix --check

[alejandrohdezma]

Checks run through .result so one failure doesn't abort the pipeline, but that also suppresses SBT's own error reporting for the wrapped task. Failures whose details only live in the exception message were reduced to N check(s) failed: ... with nothing actionable in the log — most painfully update's eviction report, where the entire conflict tree is the exception message.

Now every failed check logs its underlying causes (via Incomplete.allExceptions) prefixed with the check's name, e.g.:

[error] [compatibility] 	* com.example:library-core_2.13:2.0.0 (early-semver) is selected over 1.0.0 for compile
[error] [compatibility] 	    +- com.example:app_2.13:1.5.0 (depends on 1.0.0)
[error] [compatibility] this can be overridden using libraryDependencySchemes or evictionErrorLevel
[error] (app / fix) 1 check(s) failed: compatibility

Verified end to end against a build with a deliberately planted eviction conflict wired into fixCheckExtra as an update check. versionPolicyCheck passes (BinaryAndSourceCompatible — internal change only).