child_process: don't throw on EAGAIN

Signed-off-by: Fedor Indutny <fedor@indutny.com>

lib/child_process.js

@@ -1114,7 +1114,9 @@ ChildProcess.prototype.spawn = function(options) {
 
   var err = this._handle.spawn(options);
 
-  if (err === uv.UV_EMFILE ||
+  // Run-time errors should emit an error, not throw an exception.
+  if (err === uv.UV_EAGAIN ||
+      err === uv.UV_EMFILE ||
       err === uv.UV_ENFILE ||
       err === uv.UV_ENOENT) {
     process.nextTick(function() {