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/normalize-package-data/lib/extract_description.js

@@ -0,0 +1,14 @@
+module.exports = extractDescription
+
+// Extracts description from contents of a readme file in markdown format
+function extractDescription (d) {
+  if (!d) return;
+  if (d === "ERROR: No README data found!") return;
+  // the first block of text before the first heading
+  // that isn't the first line heading
+  d = d.trim().split('\n')
+  for (var s = 0; d[s] && d[s].trim().match(/^(#|$)/); s ++);
+  var l = d.length
+  for (var e = s + 1; e < l && d[e].trim(); e ++);
+  return d.slice(s, e).join(' ').trim()
+}