grpc-js: Propagate error messages through LB policy tree

[murgatroid99]

This modifies the ChannelControlHelper#updateState function signature to add an errorMessage argument, which can be either a string or null. The expectation is that it should be a string when the status is TRANSIENT_FAILURE, and null otherwise. The purpose of this is to enable LB policies with multiple children to provide some error information from clients when synthesizing an aggregate status to be passed to their parents. In order to avoid including excessive information in the status message, each such LB policy only passes along the last error message it has seen from any child.