Fix issue with false-positives

[ffMathy]

If there is a build error (non-zero exit code) but there are tests, it still results in a state file being persisted, with no failures in it.

That leads to the next run not executing despite the failure, since there are no GIT changes, and no tests present that are failed.

[ffMathy]

It all boils down to if the instrumentation failed.

Right now, because of using AltCover, we can't detect if something went wrong with instrumentation, or if something is wrong with the build. With Minicover, we should be able to.

A build failure will also result in no tests being run, and should then always be treated as a complete failure, with no state being persisted.

However, a build success should be treated as a normal run, if the instrumentation phase goes well. That should solve the issue.

[ffMathy]

Waiting for lucaslorentz/minicover#138 to be merged before switching over to Minicover.