Show validation message for duplicate condition names in Add/Edit Condition modal

[zackcl]

Is your feature request related to a problem? Please describe.
When adding or editing a condition using the Add/Edit Condition modal, if the typed condition name already exists in the experiment, the field becomes invalid, but no visible warning message is shown to the user (the validation message only appears in the browser console).

Describe the solution you'd like
When the Condition field is invalid because the name already exists, display a visible validation message such as:
"Condition name must be unique within the experiment."

This message should be shown instead of the generic hint text so that the user understands why the field is invalid.

Screenshot