Correct exception info format of v3 flavor manage

The existing flavor manage may raise HTTPBadRequest exception,
But the msg info didn't use '_' prefix. This patch adds it.

Change-Id: I736bf4b0e78ecc0bb753d376ee162f19fafc09e1
Log cleanups for nova.virt.libvirt.volume

More log cleanups:
 - add log hints for error and warning levels

Change-Id: Idff1575208a1004350d89848059541502ee15e9a
Merge "Log cleanups for nova.virt.libvirt.volume"
Merge "Correct exception info format of v3 flavor manage"