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/strip-bom/index.js

@@ -0,0 +1,14 @@
+'use strict';
+module.exports = x => {
+	if (typeof x !== 'string') {
+		throw new TypeError('Expected a string, got ' + typeof x);
+	}
+
+	// Catches EFBBBF (UTF-8 BOM) because the buffer-to-string
+	// conversion translates it to FEFF (UTF-16 BOM)
+	if (x.charCodeAt(0) === 0xFEFF) {
+		return x.slice(1);
+	}
+
+	return x;
+};