[Merge][#301969]Give proper error message for diff with non-existent dotted revno

[Marius Kruger]

hi all,
I added some tests for `bzr diff -r 1.1` and `bzr diff -c 1.1` where revno
1.1 does not exist,
This patch gives a consistent error message as a non-dotted revno eg. `bzr
diff -r 1234`.
It passes all tests, but I'm not sure if something else relied on getting
back (branch, None, None)
for non-existing dotted revnos.

regards
marius
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