AccountResolver2: Expose more useful exception information

Introduce a subclass of UnprocessableEntityException specific to this
resolver code. Flesh out the error message in the ambiguous and inactive
cases with the list of matching accounts, which is something end users
have wanted for a long time. One reason we weren't able to provide this
before was it was hard to reason about whether the multiple matching
accounts were visible to the calling user. Now that we have stricter
guarantees about visibility in the Result object, we know this is safe
to do.

Unfortunately, producing this exception message requires making Result
non-static so we can pass through the injected AnonymousCowardName. This
means we can't use AutoValue. However, neither downstream consumers nor
tests were depending on any of the niceties of AutoValue. Nor should
they be, really: AccountState is not immutable and doesn't implement
equals.

Rewriting the core search logic to keep track of inactive users makes it
a bit more complicated, and at this point it makes more sense to inline
the trySearch method. The complicated logic is hopefully balanced by
good test coverage.

Remove the Searcher field from Result. This wasn't providing much value
in tests, since the searcher that matched is pretty well identified by
the particular set of accounts that were returned. And now, with
inactive users being combined across searchers, there's not necessarily
a single searcher that identifies the result.

Change-Id: I500fe32ade899d991ade90f90eaf5b8b83224a0d