FIX: Don't give error 500 when invalid date param is given to admin reports (#8658)

Providing invalid dates as the end_date or start_date param causes a 500 error and creates noise in the logs. This will handle the error and returns a proper 400 response to the client with a message that explains what the problem is.

spec/requests/admin/reports_controller_spec.rb

@@ -47,6 +47,24 @@ describe Admin::ReportsController do
             expect(JSON.parse(response.body)["reports"][1]["type"]).to eq("not_found")
           end
         end
+
+        context "invalid start or end dates" do
+          it "doesn't return 500 error" do
+            get "/admin/reports/bulk.json", params: {
+              reports: {
+                topics: { limit: 10, start_date: "2015-0-1" }
+              }
+            }
+            expect(response.status).to eq(400)
+
+            get "/admin/reports/bulk.json", params: {
+              reports: {
+                topics: { limit: 10, end_date: "2015-0-1" }
+              }
+            }
+            expect(response.status).to eq(400)
+          end
+        end
       end
     end