Fix crash when $BROWSER is not set

[mgedmin]

Fixes

Traceback (most recent call last):
  File "/home/mg/.venv/bin/urlscan", line 6, in <module>
    exec(compile(open(__file__).read(), __file__, 'exec'))
  File "/home/mg/src/urlscan/bin/urlscan", line 176, in <module>
    ui.main()
  File "/home/mg/src/urlscan/urlscan/urlchoose.py", line 97, in main
    loop.run()
  File "/home/mg/.venv/local/lib/python2.7/site-packages/urwid/main_loop.py", line 286, in run
    self._run()
  File "/home/mg/.venv/local/lib/python2.7/site-packages/urwid/main_loop.py", line 379, in _run
    return self._run_screen_event_loop()
  File "/home/mg/.venv/local/lib/python2.7/site-packages/urwid/main_loop.py", line 444, in _run_screen_event_loop
    keys = self.input_filter(keys, raw)
  File "/home/mg/.venv/local/lib/python2.7/site-packages/urwid/main_loop.py", line 547, in input_filter
    return self._input_filter(keys, raw)
  File "/home/mg/src/urlscan/urlscan/urlchoose.py", line 106, in handle_keys
    if os.environ['BROWSER'] not in ['elinks', 'links', 'w3m', 'lynx']:
  File "/home/mg/.venv/lib/python2.7/UserDict.py", line 40, in __getitem__
    raise KeyError(key)
KeyError: 'BROWSER'