Gracefully handle having no default browser

[tbekolay]

From @xchoo in nengo/nengo#1478:

When nengo doesn't detect a browser it can automatically open it throws this error and quits:

Traceback (most recent call last):
  File "/home/xchoo/miniconda2/envs/inrc/bin/nengo", line 11, in <module>
    sys.exit(main())
  File "/home/xchoo/miniconda2/envs/inrc/lib/python3.5/site-packages/nengo_gui/main.py", line 123, in main
    wb = webbrowser.get()
  File "/home/xchoo/miniconda2/envs/inrc/lib/python3.5/webbrowser.py", line 51, in get
    raise Error("could not locate runnable browser")
webbrowser.Error: could not locate runnable browser

I propose changing the behaviour to run in --no-browser mode instead (similar to what jupyter notebook does when it doesn't find a runnable browser)

[xchoo]

Whoops! Hahaha. 🤣 Thanks for moving this to the right place.