Log enhancements: Alloc engine v4 alloc fail classes

After discussion with support, I was suggested to share this trivial patch here.

We are using a northbound NETCONF system to automatically fill the pool on our DHCP servers, but the standard log messages is missing vital information for us to know witch pool to fill.

I know a log parser could be created to including some caching, and somehow match transaction and/or mac addresses for the standard alloc fail messages, but Kea already has all the needed information when generating the log.

This patch was made for our specific needs, and might not have use for many others, that eg. could be using incoming interface or gi for pool selection. So would have been better suited for a hook, but log mangling is not possible via hooks.

Note this also fixed the copy paste error pointed out in #752 (closed)