Improved browser error logging in crawl_history

[englehardt]

This contains the changes in #468, will rebase once that merges.

This PR includes several improvements to error logging:

1. bool_success has been removed from the crawl_history table and replaced by command_status, which is a string.
   1. Successful commands use "ok"
   2. Exceptions in the browser manager process use "error"
   3. about:neterror WebDriverExceptions are special cased, and use "neterror"
   4. Timeouts use "timeout"
2. Error messages and full, serialized tracebacks are included in the crawl_history table in the columns error and traceback. The error message for about:neterror exceptions parses out the exact error from the about: url -- e.g., dnsError.
3. about:neterror exceptions now log at the logging.INFO level, so they should not be included in Sentry to cut down on noise.

A sample from a very small, 10 site crawl is available here: https://dbc-caf9527b-e073.cloud.databricks.com/#notebook/154881/command/154898.

[englehardt]

Now rebased on #468 and ready for review. To see what this kind of logging looks like, see: https://dbc-caf9527b-e073.cloud.databricks.com/#notebook/165717/command/166596.

[motin]

Love it :)