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/exit/test/fixtures/create-files.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+
+rm 10*.txt
+for n in 10 100 1000; do
+  node log.js 0 $n stdout stderr &> $n-stdout-stderr.txt
+  node log.js 0 $n stdout &> $n-stdout.txt
+  node log.js 0 $n stderr &> $n-stderr.txt
+done