Prevent / being used in bot or server prefixes (#5693)

* Update `[p]set prefix`/`[p]set serverprefix`

* Update cli

* style

* update __main__

* style

* improve checks

* Raise in Red.set_prefixes, update responses

* uniform responses

* Fixes

* Keep generator variable names consistent across files

redbot/core/core_commands.py

@@ -3536,6 +3536,11 @@ class Core(commands.commands._RuleDropper, commands.Cog, CoreLogic):
         **Arguments:**
             - `<prefixes...>` - The prefixes the bot will respond to globally.
         """
+        if any(prefix.startswith("/") for prefix in prefixes):
+            await ctx.send(
+                _("Prefixes cannot start with '/', as it conflicts with Discord's slash commands.")
+            )
+            return
         if any(len(x) > MAX_PREFIX_LENGTH for x in prefixes):
             await ctx.send(
                 _(
@@ -3584,6 +3589,11 @@ class Core(commands.commands._RuleDropper, commands.Cog, CoreLogic):
             await ctx.bot.set_prefixes(guild=ctx.guild, prefixes=[])
             await ctx.send(_("Server prefixes have been reset."))
             return
+        if any(prefix.startswith("/") for prefix in prefixes):
+            await ctx.send(
+                _("Prefixes cannot start with '/', as it conflicts with Discord's slash commands.")
+            )
+            return
         if any(len(x) > MAX_PREFIX_LENGTH for x in prefixes):
             await ctx.send(_("You cannot have a prefix longer than 25 characters."))
             return