I assume when you call the webservice that you fill in the values from an entity? Why not do checks on these values before you call the webservice? Otherwise create variables. Fill the variables with the values and do the checking there and call the webservice with the values from the variables.
The common practice is that the system calling the web service should do the validation (or send the message to the functional administrator).
Otherwise, remove the validation from the web service, and validate the fields inside the microflow.