HTTP API request to `/api/nodes/{node_name}` where `{node_name}` isn't a valid RabbitMQ node fails with 500 error.

[kjnilsson]

Describe the bug

HTTP API request to /api/nodes/{node_name} where {node_name} isn't a valid RabbitMQ node fails with 500 error.

A 404 would be a more appropriate response.

Reproduction steps

1. curl -v http://localhost:15672/api/nodes/NOTANODENAME'
2. observe 500 response code

...

Expected behavior

404 would be the expected response when a resource does not exist.

Additional context

rabbitmq-server/deps/rabbitmq_management/src/rabbit_mgmt_wm_node.erl

Lines 74 to 78 in 9ff387e

	find_type(Node, [{Type, Nodes} | Rest]) ->
	case lists:member(Node, Nodes) of
	true -> Type;
	false -> find_type(Node, Rest)
	end.

This function does not handle unknown node names correction, however the intire resouce_exist callback impl could be substantially optimised by just checking the the node is in rabbit_nodes:list_members/0