Fix exception_to_str to work on Python 2.x
If multiprocessing fails, don't use it during setup.
Merge pull request #2104 from mdboom/exception_to_str

Bug in cbook.exception_to_str() in v 1.2
Merge pull request #2114 from mdboom/multiprocessing-during-setup-1-2

multiprocessing use should be optional during setup
Merge pull request #1944 from mdboom/cancel-zoom

ValueError exception in drag_zoom (tk backend)