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/util.promisify/.eslintrc

@@ -0,0 +1,16 @@
+{
+	"root": true,
+
+	"extends": "@ljharb",
+
+	"env": {
+		"es6": true
+	},
+
+	"rules": {
+		"id-length": [2, { "max": 30 }],
+		"max-statements": [2, 16],
+		"no-magic-numbers": 0,
+		"operator-linebreak": [2, "before"]
+	}
+}