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.
NEXUS-6155: Comment
Merge pull request #59 from sonatype/NEXUS-6155-m2settings-constraints-and-error

NEXUS-6155: Connection constraint and error handling