Turn any environment unpickling error into IOError

[jdemeyer]

Subject: allow graceful upgrading Sphinx-1.5 -> Sphinx-1.6

Feature or Bugfix

• Bugfix

Purpose

The scenario is:

1. build some docs with Sphinx-1.5
2. upgrade to Sphinx-1.6
3. rebuild the same docs

This gives:

Traceback (most recent call last):
[...]
  File "/usr/local/src/sage-config/local/lib/python2.7/site-packages/sphinx/environment/__init__.py", line 112, in load
    env = pickle.load(f)
AttributeError: 'module' object has no attribute 'WarningStream'

A simple solution is: wrap the pickle.load(f) call in try/except and change any exception to IOError. That IOError is then caught higher up, which allows the documentation to build with a message like

loading pickled environment... failed: 'module' object has no attribute 'WarningStream'

More generally, this pickle is not crucial to building the documentation. Therefore, it should not be a hard error if this pickle is somehow corrupted.

[stephenfin]

This looks like a sane fix to me. Good work.