Catch branch creation errors for Flows

What does this MR do and why?

This change improves error handling in a service that creates Git branches for CI workloads. Previously, the code would crash with exceptions when branch creation failed. Now it returns proper error responses that can be handled gracefully by the calling code. The service also better tracks which branch was actually created and includes error logging for debugging purposes. Additionally, a small bug was fixed where code was trying to access data before checking if the operation was successful. Tests were added to verify the new error handling works correctly.

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Related to #569877