mockstore: users should not have to populate the store's map entry in order to have it return the desired error

[llorllale]

If the user sets ErrGet on this store then the store's Get() operation should immediately return with that error.

edge-core/pkg/storage/mockstore/mockstore.go

Lines 86 to 96 in 9c16791

	func (s *MockStore) Get(k string) ([]byte, error) {
	s.lock.RLock()
	defer s.lock.RUnlock()
	val, ok := s.Store[k]
	if !ok {
	return nil, storage.ErrValueNotFound
	}
	return val, s.ErrGet
	}