Fix missing flash on file edit error from web UI.

These bugs were introduced by myself at https://github.com/gitlabhq/gitlabhq/pull/7807

The fact that the refactoring lead to bugs means we should add more tests for those error cases =)

Also removed the `out[:error] = ''` which never happens: every place of the app first checks for `success`.

Added tests that would have prevented me from generating this bug.

**Needs to be double checked**

One of the tests is `@wip` as explained on the source: I can't get it working no matter what. Still it shows the behavior and does an initial step to point to the cause of the problem.

This seems to be an unrelated bug in GitLab, so I've opened a separate report at: https://github.com/gitlabhq/gitlabhq/issues/7950

**Update**

This was partially fixed after this PR at: https://github.com/gitlabhq/gitlabhq/commit/184e0f , but only partially since the blob destroy, edit and API messages were not fixed, only the new blob case.