[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/commands.py

@@ -4,12 +4,20 @@ This module contains extended classes and functions which are intended to
 replace those from the `discord.ext.commands` module.
 """
 import inspect
+from typing import TYPE_CHECKING
 
 from discord.ext import commands
 
+from .errors import ConversionFailure
+from ..i18n import Translator
+
+if TYPE_CHECKING:
+    from .context import Context
 
 __all__ = ["Command", "Group", "command", "group"]
 
+_ = Translator("commands.commands", __file__)
+
 
 class Command(commands.Command):
     """Command class for Red.
@@ -53,7 +61,7 @@ class Command(commands.Command):
         """
         Returns all parent commands of this command.
 
-        This is a list, sorted by the length of :attr:`.qualified_name` from highest to lowest. 
+        This is a list, sorted by the length of :attr:`.qualified_name` from highest to lowest.
         If the command has no parents, this will be an empty list.
         """
         cmd = self.parent
@@ -63,6 +71,37 @@ class Command(commands.Command):
             cmd = cmd.parent
         return sorted(entries, key=lambda x: len(x.qualified_name), reverse=True)
 
+    async def do_conversion(self, ctx: "Context", converter, argument: str):
+        """Convert an argument according to its type annotation.
+
+        Raises
+        ------
+        ConversionFailure
+            If doing the conversion failed.
+
+        Returns
+        -------
+        Any
+            The converted argument.
+
+        """
+        # Let's not worry about all of this junk if it's just a str converter
+        if converter is str:
+            return argument
+
+        try:
+            return await super().do_conversion(ctx, converter, argument)
+        except commands.BadArgument as exc:
+            raise ConversionFailure(converter, argument, *exc.args) from exc
+        except ValueError as exc:
+            # Some common converters need special treatment...
+            if converter in (int, float):
+                message = _('"{argument}" is not a number.').format(argument=argument)
+                raise ConversionFailure(converter, argument, message) from exc
+
+            # We should expose anything which might be a bug in the converter
+            raise exc
+
     def command(self, cls=None, *args, **kwargs):
         """A shortcut decorator that invokes :func:`.command` and adds it to
         the internal command list via :meth:`~.GroupMixin.add_command`.