Turn on warnings, resolve issues

osu.Game/Database/DatabaseContextFactory.cs

@@ -1,7 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-using System;
 using System.Linq;
 using System.Threading;
 using Microsoft.EntityFrameworkCore.Storage;
@@ -67,7 +66,7 @@ namespace osu.Game.Database
                     context = threadContexts.Value;
                 }
             }
-            catch (Exception e)
+            catch
             {
                 // retrieval of a context could trigger a fatal error.
                 Monitor.Exit(writeLock);