ui/overrides: show errors on user conflict

[Forfold]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
There was a bug with showing errors when creating a user override that would conflict with another user on the schedule. This was because we were not checking if the query was still loading before it created the errors array, so the query's data object was undefined.

Additional Info:
Converted ScheduleOverrideForm class component to a functional component for use of the relevant hooks.

[mastercactapus]

Tested locally and found a few things we can probably fix:

• When there is a conflict, the error isn't displayed unless scrolled, should render as a field error the relevant field(s)
• other errors like "cannot replace and add the same user" don't display at all, but should be in the dialog errors section (always visible)

[arurao]

LGTM