command: Don't treat config warnings as errors

Meta.backendConfig was incorrectly treating the second return value from
loadBackendConfig as if it were go "error" rather than
tfdiags.Diagnostics, which in turn meant that it would treat warnings like
errors.

This had confusing results because it still returned that
tfdiags.Diagnostics value in its own diagnostics return value, causing the
caller to see warnings even though the backendConfig function had taken
the error codepath.

command/meta_backend.go

@@ -346,9 +346,10 @@ func (m *Meta) backendConfig(opts *BackendOpts) (*configs.Backend, int, tfdiags.
 
 	if opts.Config == nil {
 		// check if the config was missing, or just not required
-		conf, err := m.loadBackendConfig(".")
-		if err != nil {
-			return nil, 0, err
+		conf, moreDiags := m.loadBackendConfig(".")
+		diags = diags.Append(moreDiags)
+		if moreDiags.HasErrors() {
+			return nil, 0, diags
 		}
 
 		if conf == nil {