function msg2id( msg ) {
	if ( !isString( msg ) ) {
		throw new TypeError( format( 'invalid argument. Must provide a string. Value: `%s`.', msg ) );
	}
	if ( !db ) {
		db = inverse( database(), {
			'duplicates': false
		});
	}
	if ( hasOwnProp( db, msg ) ) {
		return db[ msg ];
	}
	return null;
}
Returns a two-character error identifier corresponding to a provided error message.

@param {string} msg - error message
@throws {TypeError} must provide a string
@returns {(string|null)} two-character error identifier

@example
var v = msg2id( 'invalid option. `%s` option must be an array. Option: `%s`.' );
// returns 'GX'