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/function-bind/.eslintrc

@@ -0,0 +1,15 @@
+{
+	"root": true,
+
+	"extends": "@ljharb",
+
+	"rules": {
+		"func-name-matching": 0,
+		"indent": [2, 4],
+		"max-nested-callbacks": [2, 3],
+		"max-params": [2, 3],
+		"max-statements": [2, 20],
+		"no-new-func": [1],
+		"strict": [0]
+	}
+}