feat: expose errorMap on useUpdateBehaviour

The update method can be used to update more than one field for which
errors might be displayed in separate locations.

src/form.ts

-import { APIObject, ErrorDetail, RetrieveMultipleOperation, RetrieveOperation } from '@rokoli/bnb'
+import {
+  APIObject,
+  ErrorDetail,
+  ErrorMap,
+  RetrieveMultipleOperation,
+  RetrieveOperation,
+} from '@rokoli/bnb'
 import { ID, PartialUpdateOperation, useServerErrorFields, useServerErrors } from '@rokoli/bnb/drf'
 import { watchDebounced } from '@vueuse/core'
 import { sort } from 'fast-sort'
@@ -24,6 +30,7 @@ import { PickOfType } from '@/types'
 const DEFAULT_DEBOUNCE_SECONDS = 0.5
 
 type UseUpdateBehaviourResult<UpdateType> = {
+  errorMap: ComputedRef<ErrorMap>
   errors: ComputedRef<ErrorDetail[]>
   isSaving: Readonly<Ref<boolean>>
   update: UpdateType
@@ -77,6 +84,7 @@ export function useUpdateBehaviour<T extends APIObject, K extends keyof T>(
     : _update
 
   return {
+    errorMap: fieldErrorMap,
     errors,
     isSaving: readonly(isSaving),
     update,