Broken pipe should not crash the program

[kevinjqiu]

admin@localhost/test> ls | grep 
Usage: grep [OPTION]... PATTERN [FILE]...
Try 'grep --help' for more information.
Traceback (most recent call last):
  File "/home/kevin/src/cdbcli/env/bin/cdbcli", line 9, in <module>
    load_entry_point('cdbcli', 'console_scripts', 'cdbcli')()
  File "/home/kevin/src/cdbcli/env/lib/python3.5/site-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/home/kevin/src/cdbcli/env/lib/python3.5/site-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/home/kevin/src/cdbcli/env/lib/python3.5/site-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/kevin/src/cdbcli/env/lib/python3.5/site-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/home/kevin/src/cdbcli/cdbcli/main.py", line 69, in main
    r.run()
  File "/home/kevin/src/cdbcli/cdbcli/repl.py", line 103, in run
    self._run()
  File "/home/kevin/src/cdbcli/cdbcli/repl.py", line 89, in _run
    eval_(self._environment, self._couch_server, cmd_text)
  File "/home/kevin/src/cdbcli/cdbcli/repl.py", line 44, in eval_
    handler(environment=environment, couch_server=couch_server, variables=m.variables())
  File "/home/kevin/src/cdbcli/cdbcli/commands.py", line 22, in wrapper
    return fn(*args, **kwargs)
  File "/home/kevin/src/cdbcli/cdbcli/commands.py", line 76, in ls
    environment.output('{} {}'.format(type_, doc))
  File "/home/kevin/src/cdbcli/cdbcli/environment.py", line 28, in output
    self.output_stream.flush()
BrokenPipeError: [Errno 32] Broken pipe