HandlerNotFound should be published as Runtime.HandlerNotFound

[stevemao]

before

{
  "errorMessage": "RequestId: c2560dea-0377-4357-9a9d-56eb97387f97 Error: Runtime exited with error: exit status 1",
  "errorType": "Runtime.ExitError"
}

after

{
  "errorType": "Runtime.HandlerNotFound",
  "errorMessage": "Could not find handler 'handler2'."
}