[SalesForce] Can a flow produce an error message like a validation rule

The idea of an autolaunch flow is that it can complete run without user interaction. Therefore you can't show the user anything on the screen. (Your flow won't even save with an inactive screen element.)

Two options:

1) Don't make the flow autolaunch. This is possible if your account creation is always triggered from an user interaction. Then you can catch errors nicely, and the ones you miss will be shown on screen. (See: https://developer.salesforce.com/docs/atlas.en-us.salesforce_vpm_guide.meta/salesforce_vpm_guide/vpm_designer_elements_connector_fault_examples.htm )

2) You can catch the error with a fault connector and update an opportunity field with the {!$Flow.FaultMessage} variable. (I haven't tried this.) You can't catch every exception but you should be able to get most. If the user doesn't check the opportunity to often then it may be better to send him/her an email with the error message and a link to the opportunity.

You can not reference fields from lookup relations in Flow as of now. But you can create formula field in lookup object and get that formula field if you need to do it.