fix: ConnectionError no longer takes positional arguments

When storage.errors was modified to DRY up message formatting, one
line in the mongodb driver wasn't updated accordingly. This patch
updates that code so that the gate will stop breaking when it tests
against the mongodb driver.

Change-Id: I9552b2dfc2fa767815d6278eb3bc918099603c63
Merge "fix: ConnectionError no longer takes positional arguments"