For operations on Bridges API if `bridge-id` is unknown we now return `404`
instead of `400`. Also a bug was fixed that caused a crash if that was a node
operation. Additionally we now also check if the given bridge is enabled when
doing the cluster operation `start` . Affected endpoints:
 * [cluster] `/bridges/:id/:operation`,
 * [node] `/nodes/:node/bridges/:id/:operation`, where `operation` is one of
`[start|stop|restart]`.
Moreover, for a node operation, EMQX checks if node name is in our cluster and
return `404` instead of `501`.