don't print just warnings when loading backend cfg

Any warnings will be caught again when the entire config is loaded, and
duplicated in the output.

command/meta_config.go

@@ -173,10 +173,13 @@ func (m *Meta) dirIsConfigPath(dir string) bool {
 // directory even if loadBackendConfig succeeded.)
 func (m *Meta) loadBackendConfig(rootDir string) (*configs.Backend, tfdiags.Diagnostics) {
 	mod, diags := m.loadSingleModule(rootDir)
+
+	// Only return error diagnostics at this point. Any warnings will be caught
+	// again later and duplicated in the output.
 	if diags.HasErrors() {
 		return nil, diags
 	}
-	return mod.Backend, diags
+	return mod.Backend, nil
 }
 
 // loadValuesFile loads a file that defines a single map of key/value pairs.