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/lodash/_escapeStringChar.js

@@ -0,0 +1,22 @@
+/** Used to escape characters for inclusion in compiled string literals. */
+var stringEscapes = {
+  '\\': '\\',
+  "'": "'",
+  '\n': 'n',
+  '\r': 'r',
+  '\u2028': 'u2028',
+  '\u2029': 'u2029'
+};
+
+/**
+ * Used by `_.template` to escape characters for inclusion in compiled string literals.
+ *
+ * @private
+ * @param {string} chr The matched character to escape.
+ * @returns {string} Returns the escaped character.
+ */
+function escapeStringChar(chr) {
+  return '\\' + stringEscapes[chr];
+}
+
+module.exports = escapeStringChar;