When `-Ybest-effort` is used, do not spawn `.betasty` in successful compilations

[jchyb]

Compiler version

3.5.0+

Expectation

When -Ybest-effort is used, the compiler will always produce .betasty when in the pickler phase, regardless of whether the compilation up to this point had errors or not. This was so that even when the compiler fails later in the transform phases, we still have the betasty to rely on. However, this does not really make sense when considered from the build tool perspective, where in the situation where the compilation succeeds, it ends up having to clean up the files, with time wasted both on writing and subsequent deleting of the files.
We should have a separate phase for writing betasty, run directly after pickler or the first error in transform phases.