[BUG] Environment variable substitution fails silently if "__format" parser is not supported #753

Describe the bug
Defining a custom-environment-variables.json file with "__format" definitions that do not match a supported parser leads to the configuration being silently ignored.

Example:

{
  "hostname": {
    "__name": "HOSTNAME",
    "__format": "string"
  }
}

Expected behavior
An error when attempting to use an unsupported parser.

Screenshots
N/A

Please tell us about your environment:

• node-config version: 3.3.9
• node-version: 20.x

Other information

Related code:

• node-config/lib/config.js

  Lines 897 to 902 in 8c52ea2

  	util.parseString = function (content, format) {
  	const parser = Parser.getParser(format);
  	if (typeof parser === 'function') {
  	return parser(null, content);
  	}
  	};

• node-config/lib/config.js

  Lines 1103 to 1113 in 8c52ea2

  	if ('__name' in value && '__format' in value && typeof vars[value.__name] !== 'undefined' && vars[value.__name] !== '') {
  	try {
  	const parsedValue = util.parseString(vars[value.__name], value.__format);
  	} catch(err) {
  	err.message = '__format parser error in ' + value.__name + ': ' + err.message;
  	throw err;
  	}
  	util.setPath(result, pathTo.concat(prop), parsedValue);
  	} else {
  	_substituteVars(value, vars, pathTo.concat(prop));
  	}