If a Flow or process (created in Process Builder) fails at runtime or if any unhandled exception occurs, the system administrator who created the Flow or process will receive a fault e-mail from Salesforce with a fault message. The e-mail includes the error message from the failure and details about every Flow element that the interview executed. 

The preceding screenshot displays a Flow error generated at runtime because the user has entered an invalid format of an e-mail address. If the system administrator who created the Flow or process leaves the organization (that is, if the the user gets deactivated in Salesforce), and if a Flow fails at runtime or if any unhandled exception occurs, they will receive a fault e-mail from Salesforce with a fault message.

The best way to troubleshoot runtime issues is to use Debug Logs. In the debug log, you will see the Flow action events in the workflow category of Debug Logs, which shows the Flow version and the values passed into Flow variables.