return 400 when validation fails

db/index.ts

@@ -61,7 +61,12 @@ export async function createBooking({
   const bookedDays = await getBookedDays()
 
   if (booking.days.some((day: string) => bookedDays.includes(day))) {
-    throw new mongoose.Error.ValidationError(booking)
+    const error = new mongoose.Error.ValidationError(booking)
+    error.addError(
+      'endDate',
+      'At least on one day the van is not avaliable. Choose another startDate or endDate.'
+    )
+    throw error
   }
 
   let booker = await Booker.findOne({ email }).exec()