fix: raise DRF-compatible exception for configuration errors

ValueError has no built-in handling in DRF causing it to return an HTML
error page. This is unfortunate for clients that expect a JSON error
object like the dashboard.

program/exceptions.py

+from rest_framework import status
+from rest_framework.exceptions import ValidationError
+
+from django.utils.translation import gettext_lazy as _
+
+
+class ConfigurationError(ValidationError):
+    status_code = status.HTTP_500_INTERNAL_SERVER_ERROR
+    default_detail = _("Invalid or insufficient server configuration.")
+    default_code = "misconfigured"

program/services.py

@@ -30,6 +30,7 @@ from django.core.exceptions import ObjectDoesNotExist
 from django.db.models import Q, QuerySet
 from django.utils import timezone
 from django.utils.translation import gettext_lazy as _
+from program.exceptions import ConfigurationError
 from program.models import (
     Note,
     ProgramEntry,
@@ -745,7 +746,10 @@ def generate_program_entries(
     radio_settings: RadioSettings | None = RadioSettings.objects.first()
     fallback_show = radio_settings.fallback_show if radio_settings is not None else None
     if fallback_show is None:
-        raise ValueError("Radio settings must set fallback show if include_virtual is True.")
+        raise ConfigurationError(
+            "Radio settings must define a fallback show if include_virtual is True.",
+            code="no-fallback-show-defined",
+        )
 
     entry_start = start
     timeslot: TimeSlot