ENH Turn IOError into warning when cache can not be written.
ENH Turn IOError into warning when cache can not be written.

femtotrader/pyfolio,chayapan/pyfolio,r0k3/pyfolio,sketchytechky/pyfolio,ChinaQuants/pyfolio,ChinaQuants/pyfolio,quantopian/pyfolio,raybuhr/pyfolio,YihaoLu/pyfolio,yanheven/pyfolio,femtotrader/pyfolio,dursk/pyfolio,sketchytechky/pyfolio,yanheven/pyfolio,quantopian/pyfolio,raybuhr/pyfolio,r0k3/pyfolio,dursk/pyfolio,YihaoLu/pyfolio