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

Changes:
* the UI 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

Issue
https://issues.sonatype.org/browse/NEXUS-6692

CI
http://bamboo.s/browse/NX-OSSF163