[V3] Send meaningful responses on conversion failure (#1817)

* [V3] Send meaningful responses on conversion failures

* Replace existing `discord.ext.commands` imports

Just to be sure

* Better Permissions converter response

redbot/core/commands/errors.py

@@ -0,0 +1,13 @@
+"""Errors module for the commands package."""
+from discord.ext import commands
+
+__all__ = ["ConversionFailure"]
+
+
+class ConversionFailure(commands.BadArgument):
+    """Raised when converting an argument fails."""
+
+    def __init__(self, converter, argument: str, *args):
+        self.converter = converter
+        self.argument = argument
+        super().__init__(*args)