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...
