INT-3500: Consider to add "error-channel" to the <enricher>

JIRA: https://jira.spring.io/browse/INT-3500

* add 'error-channel' to the <enricher> component
* delegate errorChannel to the internal gateway in ContentEnricher
* added tests for xml based and java based enrichers with error channels

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Your issue is here. Since `replyChannel` is a `<queue>`, there is no guaranty, that message will appear in the `queue` immediately. I mean that `wait timeout` should be somewhere like `10000`.
From other side: would you mind reworking this test like real integration one? 
I mean wait result on the enricher's `output-channel`, not `reply-channel` from that downstream gateway.