Ignore permission failures from custom plumbed Matrix rooms (non-`gitter.im`) that we can't update v2

Ignore permission failures from custom plumbed Matrix rooms (non-gitter.im) that we can't update v2

Follow-up to !2382 (merged) where we tried to inspect err.errcode that didn't exist from the error thrown from matrix-appservice-bridge, see src/components/intent.ts#L1113-L1118

Part of #2857 (closed)