"Invalid filter json" error message should use an appropriate error code

[DMRobertson]

{
  "errcode": "M_UNKNOWN",
  "error": "Invalid filter JSON"
}

From

synapse/synapse/rest/client/sync.py

Lines 145 to 154 in a6895dd

	elif filter_id.startswith("{"):
	try:
	filter_object = json_decoder.decode(filter_id)
	set_timeline_upper_limit(
	filter_object, self.hs.config.server.filter_timeline_limit
	)
	except Exception:
	raise SynapseError(400, "Invalid filter JSON")
	self.filtering.check_valid_filter(filter_object)
	filter_collection = FilterCollection(self.hs, filter_object)

We should return M_NOT_JSON if the decode fails.

We should probably also defer the set_timeline_upper_limit call until self.filtering.check_valid_filter(filter_object) has passed?

[shraddhafalane]

I would like to work on it. @DMRobertson could you please elaborate more on defer logic.

[DMRobertson]

I would like to work on it. @DMRobertson could you please elaborate more on defer logic.

I was thinking that we should move lines 148-150 outside of the try block, because they do not related to the decoding of JSON. Probably between lines 153 and 154. But this isn't a hugely important change to make.