Close channels when `find_funding_output` fails to find an output

Close channels when `find_funding_output` fails to find an output

In `funding_transaction_generated_intern`, if `find_funding_output`
fails (e.g. due to the require output not being present in the
provided funding transaction) we'd previously not generated a
`ChannelClosed` event which leaves users possibly in a confused
state.

Here we fix this, also fixing the relevant tests to check for the
new event.

Fixes #2843.