Read needs a non-empty ID

[Frassle]

Description

I'm getting an internal assertion thrown here: https://github.com/pulumi/pulumi/blob/8b516bb05434ca8c6dac94b76a5583e855ffd0b6/sdk/go/common/resource/plugin/provider_plugin.go#L808
(Actually, it's line 807 in the stack trace, but presumably either the URN or ID is empty..)
This is in a brand new project/stack. Never been deployed. Any ideas on what I should be looking for? (edited) 

This was due to accidentally passing "" as the ID in a get call. This PR fixes it so we now report an error like Expected an ID for urn:pulumi:test::test::pkgA:m:typA::resA rather than a panic due to an assert.

Checklist

• I have added tests that prove my fix is effective or that my feature works

• I have updated the CHANGELOG-PENDING file with my change

• Yes, there are changes in this PR that warrants bumping the Pulumi Service API version

[iwahbe]

Improving error messages is always great. LGTM