NEXUS-6155: Connection constraint and error handling

Upped connection version constraint to 2.7+.
Removed the error handling, as it was hiding the cause.
Nexus client does good job at converting and explaining error
cause, so using the message if possible.

Issue
https://issues.sonatype.org/browse/NEXUS-6155

CI
https://bamboo.zion.sonatype.com/browse/NX-MVNF0