Workflow usage warning when deleting custom fields

Problem Definition

Users are encountering the error Unable to find field 'customfield_xxxxx' after deleting a custom field.

Suggested Solution

Add a warning before deleting a custom field when it is in used by any workflow post-function.

Why this is important

Administrator may delete the custom field without knowing that it is actually used by a workflow configuration. Users are later on blocked from creating or transitioning issues with the error Unable to find field 'customfield_xxxxx'

Workaround

To recover from the error, inspect the workflow transition that is failing and remove any post-function that has a reference to the deleted custom field.