Add exception cause support to PoolResourceLost and ConnectionLost constructors

[lachlan]

Implementing database drivers are required to raise a ConnectionLost error when they detect an existing connection has gone bad (such as when a firewall or server resets the connection). However the original exception (probably IO::Error) is lost. I'd really like to be able to see the original exception (and especially it's stack trace) which caused the ConnectionLost error to be thrown, because it will help diagnose / investigate whatever caused the issue (i.e. it could be caused by a defect in the database driver, rather than a true bad connection).

This pull request updates the PoolResourceLost (and is inherited to ConnectionLost) constructor to add the missing optional cause parameter that is available on the base Exception class.

Obviously then it will be up to the database drivers to include the cause when raising ConnectionLost.

Thanks for your consideration!

[bcardiff]

Thanks!