Fix: errors during bootstrap could still show up after bootstrap

For example, if you have a config that defines OpenGFX as baseset
but for some reason you have no basesets anymore. In that case
bootstrap downloads OpenGFX for you, but it will still show the
error that "OpenGFX was not found" after the bootstrap. This was
an error generated before the bootstrapped kicked in.

Simply muting all errors during bootstrap solves this; as we cannot
show them anyway, this is fine. Any errors that remain after
bootstrap will be generated again anyway.

src/error_gui.cpp

@@ -396,6 +396,7 @@ void ShowErrorMessage(StringID summary_msg, StringID detailed_msg, WarningLevel
 
 	bool no_timeout = wl == WL_CRITICAL;
 
+	if (_game_mode == GM_BOOTSTRAP) return;
 	if (_settings_client.gui.errmsg_duration == 0 && !no_timeout) return;
 
 	ErrorMessageData data(summary_msg, detailed_msg, no_timeout ? 0 : _settings_client.gui.errmsg_duration, x, y, textref_stack_grffile, textref_stack_size, textref_stack);