How to change the error in case of process builder if it is not able to fetch data?

You said you were using a process. Can you set the action criteria to make sure that the neccessary fields are set correctly? When using an auto launched flow, I'm not sure how to display a custom error message at save. If you can't edit your process to make sure that your flow only fires when the fields are set correctly, some more information might help. The details of the process, the way the flow works, and what objects are involved in this whole set of automation. Some of those details may help, they may not, but everyone that has an idea will know better what will and won't work, Thanks in advance.