addrs: Resource.String shouldn't ever crash

Previously we had a panic in here for invalid resource modes. While that
does always indicate a programming error, it's frustrating to have a
crasher inside a String method since it often impedes our ability to
report an error properly, since the error reporting itself can crash.

Instead we'll just return an invalid string and hope the caller really is
bailing out with an error message.

addrs/resource.go

@@ -22,7 +22,9 @@ func (r Resource) String() string {
 	case DataResourceMode:
 		return fmt.Sprintf("data.%s.%s", r.Type, r.Name)
 	default:
-		panic(fmt.Errorf("resource address with invalid mode %s", r.Mode))
+		// Should never happen, but we'll return a string here rather than
+		// crashing just in case it does.
+		return fmt.Sprintf("<invalid>.%s.%s", r.Type, r.Name)
 	}
 }