Catch RunTimeError when importing matplotlib/pylab and give reasonable error message.
Catch RunTimeError when importing matplotlib/pylab
and give reasonable error message.

Fixes #330

--HG--
extra : convert_revision : svn%3A3ed01bd8-26fb-0310-9e4c-ca1a4053419f/networkx/trunk%401595

dhimmel/networkx,michaelpacer/networkx,jni/networkx,wasade/networkx,andnovar/networkx,farhaanbukhsh/networkx,farhaanbukhsh/networkx,jtorrents/networkx,sharifulgeo/networkx,harlowja/networkx,jfinkels/networkx,JamesClough/networkx,debsankha/networkx,ghdk/networkx,jni/networkx,OrkoHunter/networkx,dmoliveira/networkx,blublud/networkx,jakevdp/networkx,RMKD/networkx,harlowja/networkx,ionanrozenfeld/networkx,bzero/networkx,bzero/networkx,ionanrozenfeld/networkx,nathania/networkx,jcurbelo/networkx,kernc/networkx,RMKD/networkx,ltiao/networkx,kai5263499/networkx,beni55/networkx,sharifulgeo/networkx,chrisnatali/networkx,aureooms/networkx,kai5263499/networkx,aureooms/networkx,debsankha/networkx,kernc/networkx,dmoliveira/networkx,nathania/networkx,nathania/networkx,dhimmel/networkx,chrisnatali/networkx,blublud/networkx,blublud/networkx,kernc/networkx,ghdk/networkx,jtorrents/networkx,harlowja/networkx,aureooms/networkx,goulu/networkx,jakevdp/networkx,yashu-seth/networkx,bzero/networkx,jni/networkx,farhaanbukhsh/networkx,ionanrozenfeld/networkx,sharifulgeo/networkx,NvanAdrichem/networkx,Sixshaman/networkx,ghdk/networkx,jakevdp/networkx,dhimmel/networkx,cmtm/networkx,kai5263499/networkx,debsankha/networkx,dmoliveira/networkx,chrisnatali/networkx,tmilicic/networkx,RMKD/networkx,SanketDG/networkx