hub: when importing greenlet causes ImportError, print the original message raised

- because the original ImportError does not necessarily complain about greenlet,
  it may also complain about setuptools (No module named pkg_resources).

gevent/hub.py

@@ -19,13 +19,19 @@ __all__ = ['getcurrent',
            'Waiter']
 
 
-try:
-    greenlet = __import__('greenlet').greenlet
-except ImportError:
+def __import_py_magic_greenlet():
     try:
         from py.magic import greenlet
+        return greenlet
     except ImportError:
-        raise ImportError('gevent requires greenlet: http://pypi.python.org/pypi/greenlet/')
+        pass
+
+try:
+    greenlet = __import__('greenlet').greenlet
+except ImportError:
+    greenlet = __import_py_magic_greenlet()
+    if greenlet is None:
+        raise
 
 getcurrent = greenlet.getcurrent
 GreenletExit = greenlet.GreenletExit