examples/ct: handler return on error
examples/ct: handler return on error

After calling http.Error, "the caller should ensure no
further writes are done to w", so return.

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