GetGCEInstanceInfo should return error if not found

`GetGCEInstanceInfo` return `nil,nil` in certain circumstances:

https://github.com/google/go-tpm-tools/blob/master/server/verify.go#L208-L211

shoudn't these `return nil, fmt.Errorf("...")`

Start at the GetGCEInstanceInfo entry point in server/verify.go around lines 208-211 and inspect the circumstances that currently return nil,nil. Confirm the expected not-found behavior and error wording from nearby code. Done means the function returns a non-nil error instead of nil,nil for that case, with the relevant tests passing.

Written by the indexing model from the issue text.