Missing error handling when creating a new branch in basic mode

[isc-pbarton]

If I create a new branch in Basic Mode, and it fails to check out the default merge branch (for example because I have some uncommitted changes that conflict), it appears to succeed. A new branch is created off of my current branch instead of the default branch.

Then when I create a merge request to the default branch on my remote, my merge includes both my commits from the old branch and the new branch.

If we fail to checkout the default merge branch, we should display an error instead of creating a new branch.