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/mkdirp/examples/pow.js

@@ -0,0 +1,6 @@
+var mkdirp = require('mkdirp');
+
+mkdirp('/tmp/foo/bar/baz', function (err) {
+    if (err) console.error(err)
+    else console.log('pow!')
+});