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/is-stream/index.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var isStream = module.exports = function (stream) {
+	return stream !== null && typeof stream === 'object' && typeof stream.pipe === 'function';
+};
+
+isStream.writable = function (stream) {
+	return isStream(stream) && stream.writable !== false && typeof stream._write === 'function' && typeof stream._writableState === 'object';
+};
+
+isStream.readable = function (stream) {
+	return isStream(stream) && stream.readable !== false && typeof stream._read === 'function' && typeof stream._readableState === 'object';
+};
+
+isStream.duplex = function (stream) {
+	return isStream.writable(stream) && isStream.readable(stream);
+};
+
+isStream.transform = function (stream) {
+	return isStream.duplex(stream) && typeof stream._transform === 'function' && typeof stream._transformState === 'object';
+};