Any error breaks shell instance

[Tristan Croll]

Working with today's daily build, if I do anything that causes any error in the ChimeraX shell:

foo()
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-3-c19b6d9633cf> in <module>
----> 1 foo()

NameError: name 'foo' is not defined

... then everything from that point on fails:

session
ERROR: execution aborted

Closing and re-opening the shell gets it running again, but loses all command history (although objects are still current).

[Tom Goddard]

Fixed.

The broken shell was running IPython 7.0.1 while older working ChimeraX shell used IPython 6.4.0. IPython comes with the PyPi QtConsole package. I've updated it from qtconsole 4.4.1 to qtconsole 4.4.2 and that newer version uses older IPython 6.4.0 which is working.