fix: raise ValidationError on invalid data

This gives the calling code better control over the handling of such
errors and makes it easy to differentiate between errors and valid
return types.

program/models.py

@@ -22,6 +22,7 @@ from datetime import datetime, time, timedelta
 
 from dateutil.relativedelta import relativedelta
 from dateutil.rrule import rrule
+from rest_framework.exceptions import ValidationError
 from versatileimagefield.fields import PPOIField, VersatileImageField
 
 from django.contrib.auth.models import User
@@ -673,17 +674,26 @@ class Schedule(models.Model):
         conflicts = Schedule.make_conflicts(sdl, schedule_pk, show_pk)
 
         if schedule.rrule.freq > 0 and schedule.first_date == schedule.last_date:
-            return {"detail": _("Start and until dates mustn't be the same")}
+            raise ValidationError(
+                _("Start and until dates mustn't be the same"),
+                code="no-same-day-start-and-end",
+            )
 
         if schedule.last_date < schedule.first_date:
-            return {"detail": _("Until date mustn't be before start")}
+            raise ValidationError(
+                _("Until date mustn't be before start"),
+                code="no-start-after-end",
+            )
 
         num_conflicts = len(
             [pr for pr in conflicts["projected"] if len(pr["collisions"]) > 0]
         )
 
         if len(solutions) != num_conflicts:
-            return {"detail": _("Numbers of conflicts and solutions don't match.")}
+            raise ValidationError(
+                _("Numbers of conflicts and solutions don't match."),
+                code="one-solution-per-conflict",
+            )
 
         # Projected timeslots to create
         create = []