hub.py: better error reporting in _import()

gevent/hub.py

@@ -231,12 +231,14 @@ def _import(path):
         raise error
     if not isinstance(path, basestring):
         return path
+    if '.' not in path:
+        raise ImportError("Cannot import %r (required format: module.class)" % path)
     module, item = path.rsplit('.', 1)
     x = __import__(module)
     for attr in path.split('.')[1:]:
         x = getattr(x, attr, _NONE)
         if x is _NONE:
-            raise ImportError('cannot import name %r from %r' % (attr, x))
+            raise ImportError('Cannot import name %r from %r' % (attr, x))
     return x