[commands] raise ConversionError on Converter error

This assumes that a Converter class raising non-CommandError
is a programmer error. Makes this type of error easier to
disambiguate from a generic BadArgument.

discord/ext/commands/core.py

@@ -212,7 +212,7 @@ class Command:
             self.instance = instance
         return self
 
-    async def do_conversion(self, ctx, converter, argument):
+    async def do_conversion(self, ctx, converter, argument, param):
         if converter is bool:
             return _convert_to_bool(argument)
 
@@ -224,21 +224,36 @@ class Command:
             if module.startswith('discord.') and not module.endswith('converter'):
                 converter = getattr(converters, converter.__name__ + 'Converter')
 
-        if inspect.isclass(converter):
-            if issubclass(converter, converters.Converter):
-                instance = converter()
-                ret = await instance.convert(ctx, argument)
-                return ret
-            else:
-                method = getattr(converter, 'convert', None)
-                if method is not None and inspect.ismethod(method):
-                    ret = await method(ctx, argument)
+        try:
+            if inspect.isclass(converter):
+                if issubclass(converter, converters.Converter):
+                    instance = converter()
+                    ret = await instance.convert(ctx, argument)
                     return ret
-        elif isinstance(converter, converters.Converter):
-            ret = await converter.convert(ctx, argument)
-            return ret
+                else:
+                    method = getattr(converter, 'convert', None)
+                    if method is not None and inspect.ismethod(method):
+                        ret = await method(ctx, argument)
+                        return ret
+            elif isinstance(converter, converters.Converter):
+                ret = await converter.convert(ctx, argument)
+                return ret
+        except CommandError as e:
+            raise e
+        except Exception as e:
+            raise ConversionError(converter) from e
+
+        try:
+            return converter(argument)
+        except CommandError as e:
+            raise e
+        except Exception as e:
+            try:
+                name = converter.__name__
+            except AttributeError:
+                name = converter.__class__.__name__
 
-        return converter(argument)
+            raise BadArgument('Converting to "{}" failed for parameter "{}".'.format(name, param.name)) from e
 
     def _get_converter(self, param):
         converter = param.annotation
@@ -268,17 +283,7 @@ class Command:
         else:
             argument = quoted_word(view)
 
-        try:
-            return (await self.do_conversion(ctx, converter, argument))
-        except CommandError as e:
-            raise e
-        except Exception as e:
-            try:
-                name = converter.__name__
-            except AttributeError:
-                name = converter.__class__.__name__
-
-            raise BadArgument('Converting to "{}" failed for parameter "{}".'.format(name, param.name)) from e
+        return (await self.do_conversion(ctx, converter, argument, param))
 
     @property
     def clean_params(self):

discord/ext/commands/errors.py

@@ -30,7 +30,7 @@ __all__ = [ 'CommandError', 'MissingRequiredArgument', 'BadArgument',
            'NoPrivateMessage', 'CheckFailure', 'CommandNotFound',
            'DisabledCommand', 'CommandInvokeError', 'TooManyArguments',
            'UserInputError', 'CommandOnCooldown', 'NotOwner',
-           'MissingPermissions', 'BotMissingPermissions']
+           'MissingPermissions', 'BotMissingPermissions', 'ConversionError']
 
 class CommandError(DiscordException):
     """The base exception type for all command related errors.
@@ -49,6 +49,19 @@ class CommandError(DiscordException):
         else:
             super().__init__(*args)
 
+class ConversionError(CommandError):
+    """Exception raised when a Converter class raises non-CommandError.
+
+    Attributes
+    ----------
+    converter: :class:`discord.ext.commands.Converter`
+        The converter that failed.
+
+    This inherits from :exc:`.CommandError`.
+    """
+    def __init__(self, converter):
+        self.converter = converter
+
 class UserInputError(CommandError):
     """The base exception type for errors that involve errors
     regarding user input.

docs/ext/commands/api.rst

@@ -187,6 +187,9 @@ Errors
 .. autoexception:: discord.ext.commands.CommandError
     :members:
 
+.. autoexception:: discord.ext.commands.ConversionError
+    :members:
+
 .. autoexception:: discord.ext.commands.MissingRequiredArgument
     :members: