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/packages/package-a/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "@my-scope/package-a",
+  "version": "0.0.0",
+  "private": true,
+  "description": "",
+  "license": "MIT",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: run tests from root\" && exit 1"
+  },
+  "dependencies": {
+    "@my-scope/package-b": "^0.0.0"
+  }
+}