Make EchoEvent::create() a no-op if the DB is read-only
Make EchoEvent::create() a no-op if the DB is read-only

This seems better for availability than stopping the world
and rolling everything back (or just throwing post-commit
errors that didn't stop the original change anyway).

Change-Id: I816b3cb5f0d26de608e620a01571a332aa832c05

wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo