Unsanitized input to str format operator

Error-reporting code in Database.command does not sanitize the string representation of the outgoing command object in the error message.

This code will raise a ValueError from the failed format rather than an OperationFailure:

import pymongo
c = pymongo.Connection()
db = c.test
db.command("%")

This is the offending code, found on line 338 in database.py:

msg = "command %r failed: %%s" % command

Replacing this line with the following line will fix the bug:

msg = "command %s failed: %%s" % repr(command).replace("%", "%%")