[Monitor] Incorrect ErrorResponse definition

Defined like this:
https://github.com/Azure/azure-rest-api-specs/blob/02a8a6b5c42725a2036afca5081af273db5fa6f0/specification/monitor/data-plane/microsoft.insights/2015-04-01/activityLogs_API.json#L368-L374

But JSON is actually:
```json
{"Code":"UnsupportedCondition","Message":"The category field is missing in the condition."}
```

This breaks all languages where the JSON lib is case sensitive (most of the language, since only C# JSON lib is case insensitive, and JSON spec itself is case sensitive...)

Should be accurate with the JSON and says `Case` and `Message`.

Breaks CLI and Python:
https://github.com/Azure/azure-sdk-for-python/issues/1556

FYI @gucalder @troydai