Returns "message" as an object instead of a string, when a BadRequest

occurs

This makes it possible for the error message to reach the client as an
object, avoiding the need for string-to-object conversions.

src/plone/rest/errors.py

@@ -6,6 +6,7 @@
 from Products.Five.browser import BrowserView
 from urllib.parse import quote
 from urllib.parse import unquote
+from zExceptions import BadRequest
 from zExceptions import NotFound
 from zope.component import adapter
 from zope.component import queryMultiAdapter
@@ -57,7 +58,10 @@ def __call__(self):
 
     def render_exception(self, exception):
         name = type(exception).__name__
-        message = str(exception)
+        if isinstance(exception, BadRequest):
+            message = exception.args[0]
+        else:
+            message = str(exception)
         result = {"type": name, "message": message}
 
         policy = queryMultiAdapter((self.context, self.request), ICORSPolicy)

src/plone/rest/testing.py

@@ -37,3 +37,12 @@ def __call__(self):
             {},
             None,
         )
+
+
+class BadRequestService(Service):
+    def __call__(self):
+        from zExceptions import BadRequest
+
+        errors = [{"error": "ValidationError", "message": "message error"}]
+
+        raise BadRequest(errors)

src/plone/rest/testing.zcml

@@ -167,4 +167,12 @@
       expose_headers="X-My-Header"
       />
 
+  <plone:service
+      method="GET"
+      factory=".testing.BadRequestService"
+      for="*"
+      permission="zope2.View"
+      name="bad-request-error"
+      />
+
 </configure>

src/plone/rest/tests/test_error_handling.py

@@ -107,3 +107,14 @@ def test_500_traceback_only_for_manager_users(self):
         self.assertRegex(
             traceback[0], r'^File "[^"]*", line \d*, in (publish|transaction_pubevents)'
         )
+
+    def test_bad_request_error_message(self):
+        response = requests.get(
+            f"{self.portal_url}/bad-request-error",
+            headers={"Accept": "application/json"},
+            auth=(SITE_OWNER_NAME, SITE_OWNER_PASSWORD),
+        )
+        self.assertEquals(
+            response.json()["message"],
+            [{"error": "ValidationError", "message": "message error"}],
+        )