improve server side validation error for double booking

db/index.ts

@@ -1,6 +1,7 @@
 import * as mongoose from 'mongoose'
 import Booker from './booker'
 import Booking from './booking'
+import { dateFormatFrontend } from '../helpers/date'
 
 let connectedPromise: Promise<typeof mongoose>
 
@@ -44,11 +45,18 @@ export async function createBooking({
   const booking = new Booking({ startDate, endDate, purpose, org, destination })
   const bookedDays = await getBookedDays()
 
-  if (booking.days.some((day: string) => bookedDays.includes(day))) {
+  const doubleBookedDays = booking.days.filter((day: string) =>
+    bookedDays.includes(day)
+  )
+  if (doubleBookedDays.length) {
     const error = new mongoose.Error.ValidationError(booking)
     error.addError(
-      'endDate',
-      new mongoose.Error.ValidatorError({ message: 'Schon gebucht' })
+      'days',
+      new mongoose.Error.ValidatorError({
+        message: `${doubleBookedDays
+          .map(dateFormatFrontend)
+          .join(', ')} schon gebucht`,
+      })
     )
     throw error
   }