Simplify test to avoid type assertion

I think the cognitive leap required by the type assertion[1] on `err`
didn't aid understanding of this test, so I've simplified the code to
remove it.

[1]: http://golang.org/ref/spec#Type_assertions
Test we call HandleChannelClose()

Test that `connection.HandleChannelClose()` is called on recoverable
errors.
Fix variable alignment

Using `go fmt`. No changes here except to whitespace.
Merge pull request #73 from alphagov/76261684-amqp-channel-closure

[#76261684] Test that HandleChannelClose() is called