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/isstream/isstream.js

@@ -0,0 +1,27 @@
+var stream = require('stream')
+
+
+function isStream (obj) {
+  return obj instanceof stream.Stream
+}
+
+
+function isReadable (obj) {
+  return isStream(obj) && typeof obj._read == 'function' && typeof obj._readableState == 'object'
+}
+
+
+function isWritable (obj) {
+  return isStream(obj) && typeof obj._write == 'function' && typeof obj._writableState == 'object'
+}
+
+
+function isDuplex (obj) {
+  return isReadable(obj) && isWritable(obj)
+}
+
+
+module.exports            = isStream
+module.exports.isReadable = isReadable
+module.exports.isWritable = isWritable
+module.exports.isDuplex   = isDuplex