NetSender client error reporting

[Alan Noble]

In addition to client-side initiated mode changes, clients also require a way to communicate errors to NetReceiver.

The proposed mechanism is an extension of that used to communicate mode changes, i.e., adding an er=ClientError query param to the /vars request.

ns.Error() string // returns the client error
ns.SetError(string) // sets the client error

[Alan Noble]

Implemented in https://bitbucket.org/ausocean/iot/commits/4beee86c2b5ff421b27ab37f568ad9d887aa78ed