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/nested_symlinks/mylib/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "mylib",
+  "version": "0.0.0",
+  "description": "",
+  "private": true,
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "buffer": "*"
+  }
+}