update error message to be consistent

ldvspec/lofardata/forms.py

@@ -23,7 +23,7 @@ class WorkSpecificationForm(ModelForm):
         self.cleaned_data = super().clean()
         self.cleaned_data["filters"] = self._extract_filters()
         if not 'obs_id' in self.cleaned_data["filters"]:
-            raise ValidationError(["You have forgotten about the 'obs_id'"])
+            raise ValidationError(["obs_id: This field is required."])
 
         return self.cleaned_data