ac4b55ed1e
This patch changes the error returned when dispatching sync IPC to a dead actor from a `MsgRouteError` error (which is handled as fatal in most error handlers) to a `MsgDropped` error (which is generally non-fatal). This is intended to reflect the effective outcome of the change, which is that the message is dropped due to the peer actor being dead. This also removes the `MsgRouteError` error type as it has been fully replaced by `MsgDropped`. The final use-case of `MsgRouteError` has been replaced by a normal assertion, as it cannot be triggered by the IPC layer. Differential Revision: https://phabricator.services.mozilla.com/D232114