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/optimist/example/usage-options.js

@@ -0,0 +1,19 @@
+var optimist = require('./../index');
+
+var argv = optimist.usage('This is my awesome program', {
+  'about': {
+    description: 'Provide some details about the author of this program',
+    required: true,
+    short: 'a',
+  },
+  'info': {
+    description: 'Provide some information about the node.js agains!!!!!!',
+    boolean: true,
+    short: 'i'
+  }
+}).argv;
+
+optimist.showHelp();
+
+console.log('\n\nInspecting options');
+console.dir(argv);