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/form-data/lib/populate.js

@@ -0,0 +1,10 @@
+// populates missing values
+module.exports = function(dst, src) {
+
+  Object.keys(src).forEach(function(prop)
+  {
+    dst[prop] = dst[prop] || src[prop];
+  });
+
+  return dst;
+};