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/abab/CHANGELOG.md

@@ -0,0 +1,24 @@
+## 2.0.0
+
+Modernization updates thanks to @TimothyGu:
+
+- Use jsdom's eslint config, remove jscs
+- Move syntax to ES6
+- Remove Babel
+- Via: https://github.com/jsdom/abab/pull/26
+
+## 1.0.4
+
+- Added license file
+
+## 1.0.3
+
+- Replaced `let` with `var` in `lib/btoa.js`
+  - Follow up from `1.0.2`
+  - Resolves https://github.com/jsdom/abab/issues/18
+
+## 1.0.2
+
+- Replaced `const` with `var` in `index.js`
+  - Allows use of `abab` in the browser without a transpilation step
+  - Resolves https://github.com/jsdom/abab/issues/15