BranchSentenceArgumentValidator#validateRepoCurrentBranchHasUpstream(args) should throw different error if repo is in detached head state (refs #16)
BranchSentenceArgumentValidator now translates its error messages
...Branch.Switch: can now deal with plain text error (refs #17)
Branch.Create: it should not offer add upstream links until that sentence is actually implemented (refs #18)
Dialog.PushBranch: it should check if repo has that local branch (refs #19)

If it does not then  the entity repository will throw error if one atempts to push a branch on multiple repos when not all of them has the branch.