Confusing error from merge-upstream when upstream tag is not found

From #bzr:

14:40 < Kamping_Kaiser> quite possible. full message is 'bzr: ERROR: Unable to find the tag for the previous upstream version, 0.20110614, in the branch: upstream-0.20110614'. i don't know how to tell if i'm in that branch; bzr info doesn't tell me

It turns out this user thought “in the branch: upstream-0.20110614” meant there was a branch with that name, which is a reasonable parsing of that text. The error should be rephrased to be less ambiguous (and so hopefully less confusing).