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/set-blocking/index.js

@@ -0,0 +1,7 @@
+module.exports = function (blocking) {
+  [process.stdout, process.stderr].forEach(function (stream) {
+    if (stream._handle && stream.isTTY && typeof stream._handle.setBlocking === 'function') {
+      stream._handle.setBlocking(blocking)
+    }
+  })
+}