GH-2731: Fix nested gateway error propagation

Fixes #2731

When we have a nested gateway configuration, we are losing the context
of the current request message in case of errors and the downstream
`MessagingException` is just re-thrown as is, without the proper
`failedMessage` and its processed `errorChannel` header.

* Check for exception type and for the `errorChannel` header in the
current `requestMessage` before re-throwing as a new
`MessageHandlingException` in the `MessagingGatewaySupport.sendAndReceive()`
* Extract `.gateway()` tests into the separate `GatewayDslTests` class

Author: artembilan