[merge] remove uses of 'assert False' to raise an exception

[Martin Pool]

There were some places in bzrlib where we said

  assert False, "something bad happened"

This is bad because assertions aren't run under python -O, and so the
interpreter would run on regardless of what was meant to be a fatal
problem.  Also in most cases there was a more useful exception we can
raise instead.

python -O is not very interesting or commonly used at the moment, but
I had a note to clean them up.

-- 
Martin
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