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/log.js

@@ -0,0 +1,25 @@
+var exit = require('../../lib/exit');
+
+var errorCode = process.argv[2];
+var max = process.argv[3];
+var modes = process.argv.slice(4);
+
+function stdout(message) {
+  if (modes.indexOf('stdout') === -1) { return; }
+  process.stdout.write('stdout ' + message + '\n');
+}
+
+function stderr(message) {
+  if (modes.indexOf('stderr') === -1) { return; }
+  process.stderr.write('stderr ' + message + '\n');
+}
+
+for (var i = 0; i < max; i++) {
+  stdout(i);
+  stderr(i);
+}
+
+exit(errorCode);
+
+stdout('fail');
+stderr('fail');