For some reason, we have two lineages of quota-related exceptions in
Nova. We have QuotaError (which sounds like an actual error), from
which all of our case-specific "over quota" exceptions inhert, such
as KeypairLimitExceeded, etc. In contrast, we have OverQuota which
lives outside that hierarchy and is unrelated. In a number of places,
we raise one and translate to the other, or raise the generic
QuotaError to signal an overquota situation, instead of OverQuota.
This leads to places where we have to catch both, signaling the same
over quota situation, but looking like there could be two different
causes (i.e. an error and being over quota).
This joins the two cases, by putting OverQuota at the top of the
hierarchy of specific exceptions and removing QuotaError. The latter
was only used in a few situations, so this isn't actually much change.
Cleaning this up will help with the unified limits work, reducing the
number of potential exceptions that mean the same thing.
Related to blueprint bp/unified-limits-nova
Change-Id: I17a3e20b8be98f9fb1a04b91fcf1237d67165871
72058b7a40