More logging for retryable MediaWiki exceptions

The api layer retries the request when it sees a MediaWiki exception
of types DBConnectionError, DBQueryError, and ReadOnlyError, and it
doesnt log the details of the request/response.  For other types of
MediaWiki exception, it only logs them, without a user-visible warning.

However as seen by bug 72850, even these retryable exceptions can be
permanent errors caused by MediaWiki bugs.  We need bug reports to
mention these MediaWiki exceptions when they occur, so this change
uses pywikibot.error like happens for all non-fatal HTTP errors
other than 503, and log the request and response.

Also use bug 62974 as a reference for DBConnectionError, rather than
two revision number references to the mediawiki code review system.

Change-Id: Ia575f87f25e38619b4a0c047b2bdb6ccc5450761