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/test/resolver/baz/package.json

@@ -0,0 +1,3 @@
+{
+    "main": "quux.js"
+}

node_modules/resolve/test/resolver/baz/quux.js

@@ -0,0 +1 @@
+module.exports = 1;