For "ordinary" errors like "file not found" for some part of the config,
we would prefer to log an error and exit without logging about a panic
and printing a stack trace.
To achieve that, we want to call `defer AuditPanic()` once, at the top
of `cmd/boulder`'s main. That's so early that we haven't yet parsed the
config, which means we haven't yet initialized a logger. We compromise:
`AuditPanic` now calls `log.Get()`, which will retrieve the configured
logger if one has been set up, or will create a default one (which logs
to stderr/stdout).
AuditPanic and Fail/FailOnError now cooperate: Fail/FailOnError panic
with a special type, and AuditPanic checks for that type and prints a
simple message before exiting when it's present.
This PR also coincidentally fixes a bug: panicking didn't previously
cause the program to exit with nonzero status, because it recovered the
panic but then did not explicitly exit nonzero.
Fixes#6933
a2b2e53045