[ENG-1825] Fix url formatting and exception handling (#9385)

## Purpose

Unfortunately it turns out our URLField were essentially broken by our custom exception handling, so that their actual message wasn't getting passed, just the int 0. This meant there was no good way of telling whether a typical 400 validation error was cause by a URLField or any other field. This PR should fix this.

## Changes

- changes to custom exception handler and add tests

## QA Notes

This is ultimately going to extended by a front-end dev who will write the exact error text message.

## Documentation

Bug fix, no docs.

## Side Effects

None that I know of.

## Ticket

https://openscience.atlassian.net/browse/ENG-1825

api/base/exceptions.py

@@ -43,11 +43,35 @@ def dict_error_formatting(errors, context, index=None):
             formatted_error_list.extend([{'source': {'pointer': '/data/{}'.format(index) + error_key}, 'detail': reason} for reason in error_description])
         elif error_key == 'non_field_errors':
             formatted_error_list.extend([{'detail': description for description in error_description}])
+        elif isinstance(error_description, list):
+            for error in error_description:
+                formatted_error_list += format_validators_errors(error, error_key, context, index)
         else:
-            formatted_error_list.extend([{'source': {'pointer': '/data/{}{}/'.format(index, get_resource_object_member(error_key, context)) + error_key}, 'detail': reason} for reason in error_description])
+            formatted_error_list += format_validators_errors(error_description, error_key, context, index)
 
     return formatted_error_list
 
+
+def format_validators_errors(error_description, error_key, context, index):
+    errors = []
+    if isinstance(error_description, ErrorDetail):
+        errors.append({
+            'source': {
+                'pointer': f'/data/{index}{get_resource_object_member(error_key, context)}/' + error_key,
+            },
+            'detail': error_description,
+        })
+    else:
+        for key, value in error_description.items():
+            errors.append({
+                'source': {
+                    'pointer': f'/data/{index}{get_resource_object_member(error_key, context)}/' + error_key,
+                },
+                'detail': value,
+            })
+
+    return errors
+
 def json_api_exception_handler(exc, context):
     """
     Custom exception handler that returns errors object as an array

api_tests/preprints/views/test_preprint_detail.py

@@ -928,6 +928,17 @@ def test_update_data_links(self, app, user, preprint, url):
         assert res.status_code == 400
         assert res.json['errors'][0]['detail'] == 'Expected a list of items but got type "str".'
 
+    @override_switch(features.SLOAN_DATA_INPUT, active=True)
+    def test_invalid_data_links(self, app, user, preprint, url):
+        preprint.has_data_links = 'available'
+        preprint.save()
+
+        update_payload = build_preprint_update_payload(preprint._id, attributes={'data_links': ['thisaintright']})
+
+        res = app.patch_json_api(url, update_payload, auth=user.auth, expect_errors=True)
+        assert res.status_code == 400
+        assert res.json['errors'][0]['detail'] == ['Enter a valid URL.']
+
     def test_update_has_prereg_links(self, app, user, preprint, url):
         update_payload = build_preprint_update_payload(preprint._id, attributes={'has_prereg_links': 'available'})
 
@@ -953,6 +964,18 @@ def test_update_has_prereg_links(self, app, user, preprint, url):
         assert log.action == PreprintLog.UPDATE_HAS_PREREG_LINKS
         assert log.params == {'value': 'available', 'user': user._id, 'preprint': preprint._id}
 
+    @override_switch(features.SLOAN_PREREG_INPUT, active=True)
+    def test_invalid_prereg_links(self, app, user, preprint, url):
+        preprint.has_prereg_links = 'available'
+        preprint.save()
+
+        update_payload = build_preprint_update_payload(preprint._id, attributes={'prereg_links': ['thisaintright']})
+
+        res = app.patch_json_api(url, update_payload, auth=user.auth, expect_errors=True)
+
+        assert res.status_code == 400
+        assert res.json['errors'][0]['detail'] == ['Enter a valid URL.']
+
     def test_update_why_no_prereg(self, app, user, preprint, url):
         update_payload = build_preprint_update_payload(preprint._id, attributes={'why_no_prereg': 'My dog ate it.'})