Bad error message when not providing required parameter to data source

[lukehoban]

This program:

import * as aws from "@pulumi/aws";
const ami = aws.getAmi({ owners: <any>undefined });
export let amiId = ami.then(a => a.id)

leads to:

Error: Invoke of 'aws:index/getAmi:getAmi' failed: undefined (undefined)

With verbose logs we see the real issue:

I0619 15:47:55.501296   17110 eventsink.go:63] eventSink::Debug(<{%reset%}>Invoke RPC finished: tok=aws:index/getAmi:getAmi; err: null, resp: ,,"owners": required field is not set<{%reset%}>)

We should be producing a useful error message here instead of undefined (undefined).

[lukehoban]

This is possibly related to https://github.com/pulumi/pulumi-terraform/issues/147, though I think it's different and possibly more important since we don't even report the error at all.