Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/decamelize/index.js

@@ -0,0 +1,13 @@
+'use strict';
+module.exports = function (str, sep) {
+	if (typeof str !== 'string') {
+		throw new TypeError('Expected a string');
+	}
+
+	sep = typeof sep === 'undefined' ? '_' : sep;
+
+	return str
+		.replace(/([a-z\d])([A-Z])/g, '$1' + sep + '$2')
+		.replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + sep + '$2')
+		.toLowerCase();
+};