NXCM-4792: Improve messaging

Made better messaging for staging errors,
together with supplying hints how to resolve them.
Somehow an enter get into here...
Fixing the build goals

That were broken by dir changes.
Refactored how "work dirs" are calculated.

to make the logic not littered across (removed
one such case) but also to make different Mojos
be able to override those.
Merge pull request #42 from sonatype/nxcm-4792-improve-messaging

NXCM-4792: Improve messaging