1.1 & 1.2 FE Enhance Error handling when enabling Secure Features via MR

When the GrapqhQl Mutation fails a meaningful error-message should be rendered to the UI.

Backend Error message: #386852 (closed)

UI-Draft: &9504 (comment 1211867922)

reproduction:

When the repository is empty:

1. Try to enable SAST from an MR : Results in a 500 response since to create a .gitlab-ci.yml with the SAST configuration, the latest commit's SHA is needed. Empty repo => no latest commit.
2. Try to enable Secret Detection from an MR : Results in a 500 response. Cause of error comes from Gitaly - attempting to create a branch in an empty repository.