PYTHON-2415 Support pickle in BulkWriteError

[ShaneHarvey]

For some background see my comment on the Jira ticket here: https://jira.mongodb.org/browse/PYTHON-2415?focusedCommentId=3476301&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-3476301

[pcorpet]

This PR broke the string representation of BulklWriteError.

str(exception)

is now

{'writeErrors': [{'index': 1, 'code': 11000, 'op': {'_id': 1}, 'errmsg': 'E11000 Duplicate Key Error'}], 'nInserted': 1}

Whereas it should be (at least I think so):

batch op errors occurred, full error: {'writeErrors': [{'index': 1, 'code': 11000, 'op': {'_id': 1}, 'errmsg': 'E11000 Duplicate Key Error'}], 'nInserted': 1}

[pcorpet]

I've created #522 to forward fix.