maintner: log about loading unknown JSON only in verbose mode
maintner: log about loading unknown JSON only in verbose mode

We already log about storing unknown JSON (on the server) and can look
up occurrences there. Most clients don't care, so only log if they are
in verbose mode.

Fixes golang/go#46180.

Change-Id: I665511e3d69827108bf59ddcd5e44359cacbaaa2
Reviewed-on: https://go-review.googlesource.com/c/build/+/320290
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

golang/build,golang/build,golang/build,golang/build