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/remove-trailing-separator/index.js

@@ -0,0 +1,17 @@
+var isWin = process.platform === 'win32';
+
+module.exports = function (str) {
+	var i = str.length - 1;
+	if (i < 2) {
+		return str;
+	}
+	while (isSeparator(str, i)) {
+		i--;
+	}
+	return str.substr(0, i + 1);
+};
+
+function isSeparator(str, i) {
+	var char = str[i];
+	return i > 0 && (char === '/' || (isWin && char === '\\'));
+}