Nested error JSON

This issue has been migrated from [#3054](https://github.com/matrix-org/synapse/issues/3054).

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The 429 error returned from `/room/{id}/send/m.room.message/` is accompanied by the JSON body nested as follows: `{"errcode":"M_UNKNOWN","error":"{\"errcode\":\"M_LIMIT_EXCEEDED\",\"error\":\"Too Many Requests\",\"retry_after_ms\":4079}"}`. The request URL and the response to it can be seen in the Travis build of libqmatrixclient (failing for other reasons): https://travis-ci.org/QMatrixClient/libqmatrixclient/jobs/360536201#L1281. Fortunately the HTTP code is correct.

Possibly related to #2322.