Show traceback, continuing to start kernel if pylab init fails
Show traceback, continuing to start kernel if pylab init fails

Error message goes directly over stderr (and traceback over stdout),
rather than as a pyerr message, which is associated with a particular
frontend, and thus ignored because startup actions were not requested
by anyone.

This better matches the terminal behavior, where `ipython --pylab` without matplotlib
installed shows a traceback, but continues to start.

In the qtconsole the traceback will come in below the banner, but in the notebook
the message will not arrive until after the first execution.

closes gh-990, closes gh-991.

ipython/ipython,ipython/ipython