Log root priming failures at severity NOTICE

Resolver root priming is logged at level INFO.

From BIND 9.18 and newer, root priming also logs the outcome (success or failure).

With our interest in #2744 (closed) continuing, it becomes useful to know if this condition (logged from from prime_done()) occurred because root priming failed, or because it succeeded but before we ran checkhints, the newly primed roots were no longer in cache any more (or perhaps there had been a problem writing them there?).

However, much much stuff is logged at level INFO in category Resolver and it's not uncommon for administrators to set the logging level higher for this category so that they're not overwhelmed by it.

Since logging of priming is (quite reasonably) logged at INFO because it's a routine operation, we would like to request that logging of the failure to prime, be logged at NOTICE since this exceptional.

This will also make it easier to catch this logged message on a busy resolver (for it is the busy ones that most often encounter #2744 (closed) )

See Support ticket #20909 for an example of why we need this.