gcs: Check if the given project id argument exists (#4583)

Using GCS resource manager API, check if the provided
project id is already created and associated to the current
user account.

cmd/gateway-gcs-errors.go

@@ -19,9 +19,12 @@ package cmd
 import "errors"
 
 var (
-	// ProjectID format is not valid.
+	// Project ID format is not valid.
 	errGCSInvalidProjectID = errors.New("GCS project id is either empty or invalid")
 
+	// Project ID not found
+	errGCSProjectIDNotFound = errors.New("unknown project id")
+
 	// Multipart identifier is not in the correct form.
 	errGCSNotValidMultipartIdentifier = errors.New("Not a valid multipart identifier")
 )