Address further fixes in bitmessagecli.py

[coffeedogs]

Once all automated fixers have been implemented in the tools (superfluous-parens, E712), re-base, re-run the fixers then do these manual changes.

• It seems os._exit() may be removed
• Maybe os.path.join(directory, fileName) would be better than filePath = directory + '/' + fileName
• Use with open() context manager where possible