Allow "resurrection" of MMS branches via MDK

[wobrschalek]

When I branch gets deleted via VE or API, and a new commit of the branch is done from MDK a server 500 error is returned

[dlamoris]

branch 'resurrection' is a bit complicated since we dont' know whether we just 'resurrect' or it should really be a delete and rebranch from parent's latest, needs more consideration, for now we'll just update the error message to be more informative

[dlamoris]

reopening as fix was better error message, real fix needs more discusison