server: make validation errors more specific
server: make validation errors more specific

Particularly for the validating client/, which does several operations
under the covers for a single API call -- so having more of a clue as
to which one failed is helpful.

gdbelvin/trillian,AlCutter/trillian,codingllama/trillian,daviddrysdale/trillian,Martin2112/trillian,daviddrysdale/trillian,gdbelvin/trillian,AlCutter/trillian,gdbelvin/trillian,Martin2112/trillian,gtank/trillian,gtank/trillian,google/trillian,daviddrysdale/trillian,RJPercival/trillian,Martin2112/trillian,google/trillian,codingllama/trillian,RJPercival/trillian