[FIXED JENKINS-53804] Call post conditions as early as possible

[abayer]

• JENKINS issue(s):
  • JENKINS-53804
• Description:
  • Specifically, try to catch errors from directives and execute the post conditions for the build/stage right afterwards, while we're still inside any enclosing directives - i.e., still in top-level agent when a top-level credentials explodes.
  • Also reworked post handling in general for stages when I realized that an error in a directive before execution of a stage's steps or nested stages would actually result in post for the stage not being run at all, which is not exactly ideal.
• Documentation changes:
  • n/a
• Users/aliases to notify:
  • ...

[abayer]

Note that I've put this in work-in-progress for the moment, because at a minimum I need to add javadoc where I've left TODOs, and in general...I'm just not sure yet about this one.

[abayer]

...and there are a pile o' test failures, suggesting I screwed something else up. =)

[abayer]

Yup, something's wrong with how stage errors are logged, at a minimum. I'll pick this up either tomorrow or after Jenkins World Nice next week.

[dwnusbaum]

Would it be useful to track these exceptions as suppressed exceptions rather than just dropping them, or are exceptions after the first one generally useless to the end user? I am imagining something like:

if (firstErrorSeen == null) {
    firstErrorSeen = e
} else {
    firstErrorSeen.addSuppressed(e)
}
try { 
    executePostBuild(root)
} catch (Throwable e2) {
    firstErrorSeen.addSuppressed(e2)
}

[bitwiseman]

@abayer Any chance you can resolve the merge conflicts here? Should this remain open?