Event creation missing field error is not clear enough

**Describe the bug**
When creating an event and a field hasn't been filled in (image for example) the error is not clear about what is missing.

**To Reproduce**
Steps to reproduce the behavior:
1. Create new event
2. Leave image field empty
3. Press create
4. See error, page scrolls up but does not say what is missing

**Expected behavior**
There should be an indication text showing what is missing

Reproduce the event creation flow by leaving the image field empty and submitting the form. Trace the validation and error display used by that flow; done means the resulting message identifies the missing field and the behavior is covered by the relevant existing checks.

Written by the indexing model from the issue text.