This now gives an error instead of a panic when encountering
configuration such as described in hashicorp/terraform#5740:
```
resource "aws" "web" {
provider = "aws" {
region = "us-west-2"
}
}
```
We now return an error message - "hcl object keys must be a string"
instead of crashing.
Fixeshashicorp/terraform#5740.
