Show error message when start and finish date is invalid

bookwyrm/forms.py

@@ -480,6 +480,16 @@ class SortListForm(forms.Form):
     )
 
 class ReadThroughForm(CustomForm):
+    def clean(self):
+        """make sure the email isn't in use by a registered user"""
+        cleaned_data = super().clean()
+        start_date = cleaned_data.get("start_date")
+        finish_date = cleaned_data.get("finish_date")
+        if start_date > finish_date:
+            self.add_error(
+                "finish_date", _("Reading finish date cannot be before start date.")
+            )
+
     class Meta:
         model = models.ReadThrough
         fields = ["user", "book", "start_date", "finish_date"]