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/mimic-fn/index.js

@@ -0,0 +1,13 @@
+'use strict';
+
+const mimicFn = (to, from) => {
+	for (const prop of Reflect.ownKeys(from)) {
+		Object.defineProperty(to, prop, Object.getOwnPropertyDescriptor(from, prop));
+	}
+
+	return to;
+};
+
+module.exports = mimicFn;
+// TODO: Remove this for the next major release
+module.exports.default = mimicFn;