Improve error message when a branch name does not match the regular expression in the branch name setting.

Problem

When a branch name rule is set...

... and one attempts to create a new branch but does not match the regular expression in the rule, then the error message does not explain that this is the problem:

Proposal

IF THE BRANCH CREATION FAILS BECAUSE THE BRANCH NAME ENTERED IS NOT IN LINE WITH THE BRANCH NAME RULE offer an error message such as:

Can't create branch. The branch name must match this regular expression: [regex goes here] (<- show the actual regular expression so users can fix the name themselves even if they don't have access to the settings.)

Availability and Testing

Exploratory and regression testing in the MR, please assign a SET for a review.