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/json-stringify-safe/stringify.js

@@ -0,0 +1,27 @@
+exports = module.exports = stringify
+exports.getSerialize = serializer
+
+function stringify(obj, replacer, spaces, cycleReplacer) {
+  return JSON.stringify(obj, serializer(replacer, cycleReplacer), spaces)
+}
+
+function serializer(replacer, cycleReplacer) {
+  var stack = [], keys = []
+
+  if (cycleReplacer == null) cycleReplacer = function(key, value) {
+    if (stack[0] === value) return "[Circular ~]"
+    return "[Circular ~." + keys.slice(0, stack.indexOf(value)).join(".") + "]"
+  }
+
+  return function(key, value) {
+    if (stack.length > 0) {
+      var thisPos = stack.indexOf(this)
+      ~thisPos ? stack.splice(thisPos + 1) : stack.push(this)
+      ~thisPos ? keys.splice(thisPos, Infinity, key) : keys.push(key)
+      if (~stack.indexOf(value)) value = cycleReplacer.call(this, key, value)
+    }
+    else stack.push(value)
+
+    return replacer == null ? value : replacer.call(this, key, value)
+  }
+}