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/resolve-url/component.json

@@ -0,0 +1,15 @@
+{
+  "name": "resolve-url",
+  "version": "0.2.1",
+  "license": "MIT",
+  "description": "Like Node.js’ `path.resolve`/`url.resolve` for the browser.",
+  "main": "resolve-url.js",
+  "repo": "lydell/resolve-url",
+  "keywords": [
+    "resolve",
+    "url"
+  ],
+  "scripts": [
+    "resolve-url.js"
+  ]
+}