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/_baseIteratee.js

@@ -0,0 +1,31 @@
+var baseMatches = require('./_baseMatches'),
+    baseMatchesProperty = require('./_baseMatchesProperty'),
+    identity = require('./identity'),
+    isArray = require('./isArray'),
+    property = require('./property');
+
+/**
+ * The base implementation of `_.iteratee`.
+ *
+ * @private
+ * @param {*} [value=_.identity] The value to convert to an iteratee.
+ * @returns {Function} Returns the iteratee.
+ */
+function baseIteratee(value) {
+  // Don't store the `typeof` result in a variable to avoid a JIT bug in Safari 9.
+  // See https://bugs.webkit.org/show_bug.cgi?id=156034 for more details.
+  if (typeof value == 'function') {
+    return value;
+  }
+  if (value == null) {
+    return identity;
+  }
+  if (typeof value == 'object') {
+    return isArray(value)
+      ? baseMatchesProperty(value[0], value[1])
+      : baseMatches(value);
+  }
+  return property(value);
+}
+
+module.exports = baseIteratee;