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
* aligned regex in legacy and rapture UI
Merge pull request #635 from sonatype/NEXUS-6692-fix-url

REVIEW NEXUS-6692:  Fix url validation and error handling