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/multirepo/package.json

@@ -0,0 +1,20 @@
+{
+  "name": "monorepo-symlink-test",
+  "private": true,
+  "version": "0.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "postinstall": "lerna bootstrap",
+    "test": "node packages/package-a"
+  },
+  "author": "",
+  "license": "MIT",
+  "dependencies": {
+    "jquery": "^3.3.1",
+    "resolve": "../../../"
+  },
+  "devDependencies": {
+    "lerna": "^3.4.3"
+  }
+}