ConnectionPoolMBeanManager logs an error if connection pool has been unregistered twice

[marccarre]

Given a scenario where ConnectionPool<?> is closed twice for some reason, an error is logged by ConnectionPoolMBeanManager with simply the MBean name as the message (see also #558).

Indeed since the MBean has already been unregistered, the second time the code is run, an InstanceNotFoundException is thrown. This, however, should probably be logged at an INFO or WARN level to avoid any confusion.