Don't crash when starting notebook server if runnable browser not found

[wesm]

Parking this issue here as it's become very annoying on my Kubuntu system with google-chrome installed. I'll make a PR if no one else gets to it, but I think this should get fixed for 0.13

11:03 ~/code/ipython  (master)$ ipython notebook --pylab=inline
[NotebookApp] Using existing profile dir: u'/home/wesm/.config/ipython/profile_default'
[NotebookApp] Serving notebooks from /home/wesm/code/ipython
[NotebookApp] The IPython Notebook is running at: http://127.0.0.1:8888/
[NotebookApp] Use Control-C to stop this server and shut down all kernels.
Traceback (most recent call last):
  File "/home/wesm/epd/bin/ipython", line 9, in <module>
    load_entry_point('ipython==0.13.dev', 'console_scripts', 'ipython')()
  File "/home/wesm/code/ipython/IPython/frontend/terminal/ipapp.py", line 389, in launch_new_instance
    app.start()
  File "/home/wesm/code/ipython/IPython/frontend/terminal/ipapp.py", line 359, in start
    return self.subapp.start()
  File "/home/wesm/code/ipython/IPython/frontend/html/notebook/notebookapp.py", line 557, in start
    browser = webbrowser.get()
  File "/home/wesm/epd/lib/python2.7/webbrowser.py", line 52, in get
    raise Error("could not locate runnable browser")
Error: could not locate runnable browser

If you suspect this is an IPython bug, please report it at:
    https://github.com/ipython/ipython/issues
or send an email to the mailing list at ipython-dev@scipy.org

You can print a more detailed traceback right now with "%tb", or use "%debug"
to interactively debug it.

Extra-detailed tracebacks for bug-reporting purposes can be enabled via:
    c.Application.verbose_crash=True

This also doesn't work:

11:05 ~/code/ipython  (master)$ ipython notebook --pylab=inline --browser=google-chrome[NotebookApp] Using existing profile dir: u'/home/wesm/.config/ipython/profile_default'
[NotebookApp] Serving notebooks from /home/wesm/code/ipython
[NotebookApp] The IPython Notebook is running at: http://127.0.0.1:8888/
[NotebookApp] Use Control-C to stop this server and shut down all kernels.
Traceback (most recent call last):
  File "/home/wesm/epd/bin/ipython", line 9, in <module>
    load_entry_point('ipython==0.13.dev', 'console_scripts', 'ipython')()
  File "/home/wesm/code/ipython/IPython/frontend/terminal/ipapp.py", line 389, in launch_new_instance
    app.start()
  File "/home/wesm/code/ipython/IPython/frontend/terminal/ipapp.py", line 359, in start
    return self.subapp.start()
  File "/home/wesm/code/ipython/IPython/frontend/html/notebook/notebookapp.py", line 555, in start
    browser = webbrowser.get(self.browser)
  File "/home/wesm/epd/lib/python2.7/webbrowser.py", line 52, in get
    raise Error("could not locate runnable browser")
Error: could not locate runnable browser

If you suspect this is an IPython bug, please report it at:
    https://github.com/ipython/ipython/issues
or send an email to the mailing list at ipython-dev@scipy.org

You can print a more detailed traceback right now with "%tb", or use "%debug"
to interactively debug it.

Extra-detailed tracebacks for bug-reporting purposes can be enabled via:
    c.Application.verbose_crash=True

[fperez]

Yup, crash bug is always a blocker. Thanks to @bfroehle for the quick response, I'll go ahead with review now.

[fperez]

@wesm, we just merged the fix by merging #2010, thanks for reporting.