Disallow switching to member in another system

PluralKit.Bot/Commands/SwitchCommands.cs

@@ -27,6 +27,11 @@ namespace PluralKit.Bot.Commands
 
         private async Task DoSwitchCommand(ICollection<PKMember> members)
         {
+            // Make sure all the members *are actually in the system*
+            // PKMember parameters won't let this happen if they resolve by name
+            // but they can if they resolve with ID
+            if (members.Any(m => m.System != Context.SenderSystem.Id)) throw Errors.SwitchMemberNotInSystem;
+            
             // Make sure there are no dupes in the list
             // We do this by checking if removing duplicate member IDs results in a list of different length 
             if (members.Select(m => m.Id).Distinct().Count() != members.Count) throw Errors.DuplicateSwitchMembers;

PluralKit.Bot/Errors.cs

@@ -49,6 +49,7 @@ namespace PluralKit.Bot {
         }
 
         public static PKError DuplicateSwitchMembers => new PKError("Duplicate members in member list.");
+        public static PKError SwitchMemberNotInSystem => new PKError("One or more switch members aren't in your own system.");
 
         public static PKError InvalidDateTime(string str) => new PKError($"Could not parse '{str}' as a valid date/time.");
         public static PKError SwitchTimeInFuture => new PKError("Can't move switch to a time in the future.");