We were throwing two different errors for the same problematic:
* "Expected type {expected], got {type}" --> RPCTypeCheckArgument()
* "JSON value of type {type} is not of expected type {expected}" --> UniValue::checkType()
This adds light functional coverage to estimaterawfee - a subset of
the testing applied to estimatesmartfee, and argument validation
testing to both estimaterawfee and estimatesmartfee.
One valid estimatesmartfee signature test is commented out because it
fails currently.