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/debug/.eslintrc

@@ -0,0 +1,11 @@
+{
+  "env": {
+    "browser": true,
+    "node": true
+  },
+  "rules": {
+    "no-console": 0,
+    "no-empty": [1, { "allowEmptyCatch": true }]
+  },
+  "extends": "eslint:recommended"
+}