Continue to next item if already crawled

There's nothing further we need to do if a URL has already been crawled,
so continue to the next item.
Flip 'URL exists' logic for readability

Adjust the `if` statement to avoid the negative evaluation,
keep error checking code together and separate out the positive action,
`outbound <- message`.
Continue on error
Simplify if statement
Merge pull request #43 from alphagov/tidy-continues

Tidy continues and simplify some `if` blocks