Unicode Error traceback when supplying non ascii command or option

Reported by Alexander Belchenko: If you try to supply a unicode command name or command line option, bzr aborts with a traceback, rather than a nice error.

Steps to reproduce: just run bzr with a non-existance unicode command:

$ bzr جوجو

Gives this traceback:
bzr: ERROR: exceptions.UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-3: ordinal not in range(128)

Traceback (most recent call last):
  File "/home/jameinel/dev/bzr/bzr.dev/bzrlib/commands.py", line 606, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/home/jameinel/dev/bzr/bzr.dev/bzrlib/commands.py", line 552, in run_bzr cmd = str(argv.pop(0))
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-3: ordinal not in range(128)

bzr 0.10.0dev0 on python 2.4.3.final.0 (linux2)
arguments: ['/home/jameinel/bin/bzr', '\xd8\xac\xd9\x88\xd8\xac\xd9\x88']

** please send this report to <email address hidden>

While we probably won't explicitly support unicode commands or options, we at least can produce a nicer error message than a full traceback.