fix: smarter handling of exit code 2

Only exit after code 2 if it happened quickly; improve messaging (recommend `||exit 1`)

If nodemon exits _quickly_ on code 2, then it'll bail and warn. If it's after ~500ms, then it assumes that the sub-process intentionally used exit 2 (like mocha or stylelint), it won't bail and treat it as a failure (like a crash).

Props to @joeytwiddle

Fixes #496 
Fixes #627

lib/monitor/run.js

@@ -158,9 +158,22 @@ function run(options) {
       process.exit();
     }
 
-    if (code === 2) {
-      // something wrong with parsed command
+    // If the command failed with code 2, it may or may not be a syntax error
+    // See: http://git.io/fNOAR
+    // We will only assume a parse error, if the child failed quickly
+    if (code === 2 && Date.now() < config.lastStarted + 500) {
       utils.log.error('process failed, unhandled exit code (2)');
+      utils.log.error('');
+      utils.log.error('Either the command has a syntax error,');
+      utils.log.error('or it is exiting with reserved code 2.');
+      utils.log.error('');
+      utils.log.error('To keep nodemon running even after a code 2,');
+      utils.log.error('add this to the end of your command: || exit 1');
+      utils.log.error('');
+      utils.log.error('Read more here: https://git.io/fNOAG');
+      utils.log.error('');
+      utils.log.error('nodemon will stop now so that you can fix the command.');
+      utils.log.error('');
       bus.emit('error', code);
       process.exit();
     }