NEXUS-6094: Staging Mojos made aware of build failures

As in case of -fae switch, Mojo might be invoked to stage
even if no staging should occur.
NEXUS-6094: Staging Mojos made aware of build failures

Reworked and simplified previous commit, and added user ability
to control failure detection.
Merge remote-tracking branch 'origin/master' into NEXUS-6094-be-aware-of-build-failures
NEXUS-6094: Added IT
NEXUS-6094: Be safe with Maven2

That lacks this method from MavenSession.
NEXUS-6094: Moved failure to another module

As it was the last module that was failing, and
hence the fix was not kicking in, as the build
was anyway about to be aborted.
Merge remote-tracking branch 'origin/master' into NEXUS-6094-be-aware-of-build-failures

Conflicts:
	staging/maven-plugin/src/main/java/org/sonatype/nexus/maven/staging/AbstractStagingMojo.java
NEXUS-6094 - adjust log messages and parameter description to be simpler and hopefully clearer
Merge pull request #57 from sonatype/NEXUS-6094-be-aware-of-build-failures

NEXUS-6094: Staging Mojos made aware of build failures