PYTHON-2342 Prefer checking error codes over error messages

[ShaneHarvey]

I've performed the following requests in the drivers ticket:

• Audit the driver for uses of error messages to make decisions
• For each error message, determine the server's related error code and check that first, falling back to error message matching to avoid breaking changes with old server versions.

For example, the Python driver does an exact match on the error message "ns not found". It should attempt to match error code 26 first, matching the error message as a fallback. The fallback is necessary to avoid a backward breaking change with MongoDB 2.6 and 3.0 which do not return an error code.

I also removed the "db assertion failure" check which was removed by the server in MongoDB 2.5.

[ShaneHarvey]

Enabling "Hide whitespace changes" (w=1 in the url) makes this change easier to review: https://github.com/mongodb/mongo-python-driver/pull/492/files?diff=split&w=1

[ShaneHarvey]

msg was never used.

[ShaneHarvey]

This is ready for review. I had to revert the PYTHON-2381 changes because of backwards compatibility concerns with 2.6/3.0.