Expose SERVER_DOWN if connection fails

Without this change, ldappool might fail to bind to an LDAP server for
any number of reasons, but eats most of them and instead raises its own
not-very-descriptive error. A network failure or the LDAP server being
down is useful information that a user might be able to do something
with (see this related patch[1]) so let's add that to the list of
exceptions ldappool raises directly.

[1] https://review.openstack.org/#/c/390948

Change-Id: Iebb58f9046707a044ed86d8ce940e4c230ffd2aa

ldappool/__init__.py

@@ -267,7 +267,8 @@ class ConnectionManager(object):
 
         if not connected:
             if isinstance(exc, (ldap.NO_SUCH_OBJECT,
-                                ldap.INVALID_CREDENTIALS)):
+                                ldap.INVALID_CREDENTIALS,
+                                ldap.SERVER_DOWN)):
                 raise exc
 
             # that's something else

ldappool/tests/test_ldapconnection.py

@@ -133,7 +133,7 @@ class TestLDAPConnection(unittest.TestCase):
         try:
             with cm.connection('dn', 'pass'):
                 pass
-        except ldappool.BackendError:
+        except ldap.SERVER_DOWN:
             pass
         else:
             raise AssertionError()