GetTokenFromCLIWithParams fixed empty error message if command invoke…

… failed. (#653)

* Fixed empty error message if cli invoke failed

* Addendum to last commit

* Check stderr for empty string

Co-authored-by: Joel Hendrix <jhendrix@microsoft.com>

autorest/azure/cli/token.go

@@ -170,7 +170,11 @@ func GetTokenFromCLIWithParams(params GetAccessTokenParams) (*Token, error) {
 
 	output, err := cliCmd.Output()
 	if err != nil {
-		return nil, fmt.Errorf("Invoking Azure CLI failed with the following error: %s", stderr.String())
+		if stderr.Len() > 0 {
+			return nil, fmt.Errorf("Invoking Azure CLI failed with the following error: %s", stderr.String())
+		}
+
+		return nil, fmt.Errorf("Invoking Azure CLI failed with the following error: %s", err.Error())
 	}
 
 	tokenResponse := Token{}