api/deploy: changes rollbackUpdate error handling
api/deploy: changes rollbackUpdate error handling

    This warns the user of a conflict while trying to update
    the rollback of a version of a app, instead of throwing just
    a `not found` http code.

ArxdSilva/tsuru,guilhermebr/tsuru,cezarsa/tsuru,ArxdSilva/tsuru,cezarsa/tsuru,tsuru/tsuru,guilhermebr/tsuru,ArxdSilva/tsuru,guilhermebr/tsuru,ggarnier/tsuru,ArxdSilva/tsuru,tsuru/tsuru,tsuru/tsuru,cezarsa/tsuru,cezarsa/tsuru,ggarnier/tsuru,ggarnier/tsuru,guilhermebr/tsuru,tsuru/tsuru,ggarnier/tsuru