NEXUS-6692: Fix url validation and error handling

Changes:
* the regex was too strict, not allowing port numbers nor localhost with paths
* on repo creation, the validation error was swallowed (wrapped into another config ex)
* on repo update the validation was not handled resulting in generic config error
NEXUS-6692: 2.9 UI URL validation made back to original
Merge pull request #634 from sonatype/nexus-2.9.x-NEXUS-6692-fix-url

REVIEW [2.9] NEXUS-6692: Fix url validation and error handling