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/es-abstract/test/.eslintrc

@@ -0,0 +1,13 @@
+{
+	"rules": {
+		"id-length": 0,
+		"max-lines": 0,
+		"max-lines-per-function": 0,
+		"max-statements-per-line": [2, { "max": 3 }],
+		"max-nested-callbacks": [2, 4],
+		"max-statements": 0,
+		"no-implicit-coercion": 1,
+		"no-invalid-this": 1,
+		"object-curly-newline": 0,
+	}
+}