config: don't log to stderr if lnd.conf isn't found

This commit fixes a bit of a wart in the configuration file handling
wherein if the config file isn’t found, an error was printed to stderr.
At times, this would cause the testing framework to erroneously mark as
test as failed. Instead, we now keep a particular config file error,
and check that, printing to a warning log level.

config.go

@@ -159,9 +159,10 @@ func loadConfig() (*config, error) {
 	}
 
 	// Next, load any additional configuration options from the file.
+	var configFileError error
 	cfg := defaultCfg
 	if err := flags.IniParse(preCfg.ConfigFile, &cfg); err != nil {
-		fmt.Fprintln(os.Stderr, err)
+		configFileError = err
 	}
 
 	// Finally, parse the remaining command line options again to ensure
@@ -299,6 +300,13 @@ func loadConfig() (*config, error) {
 		return nil, err
 	}
 
+	// Warn about missing config file only after all other configuration is
+	// done.  This prevents the warning on help messages and invalid
+	// options.  Note this should go directly before the return.
+	if configFileError != nil {
+		ltndLog.Warnf("%v", configFileError)
+	}
+
 	return &cfg, nil
 }