Kaizen: Make error_handler.py more embeddy.

Currently, some types of errors are returning plain strings that repeat
the input (which can be exploited to deliver stuff like mentions), and
others are returning generic messages that don't give any exception
information.

This commit unifies our approach around putting as much information as
we can (including the exception message), but always putting it inside
an embed, so that stuff like pings will not fire.

This, combined with the 1.4.0a `allowed_mentions` functionality, seems
like a reasonable compromise between security and usability.

1 files changed, 25 insertions, 7 deletions

diff --git a/bot/cogs/error_handler.py b/bot/cogs/error_handler.py
index 233851e41..f9d4de638 100644
--- a/bot/cogs/error_handler.py
+++ b/bot/cogs/error_handler.py
@@ -2,12 +2,13 @@ import contextlib
 import logging
 import typing as t
 
+from discord import Embed
 from discord.ext.commands import Cog, Context, errors
 from sentry_sdk import push_scope
 
 from bot.api import ResponseCodeError
 from bot.bot import Bot
-from bot.constants import Channels
+from bot.constants import Channels, Colours
 from bot.converters import TagNameConverter
 from bot.utils.checks import InWhitelistCheckFailure
 
@@ -20,6 +21,14 @@ class ErrorHandler(Cog):
     def __init__(self, bot: Bot):
         self.bot = bot
 
+    def _get_error_embed(self, title: str, body: str) -> Embed:
+        """Return an embed that contains the exception."""
+        return Embed(
+            title=title,
+            colour=Colours.soft_red,
+            description=body
+        )
+
     @Cog.listener()
     async def on_command_error(self, ctx: Context, e: errors.CommandError) -> None:
         """
@@ -162,25 +171,34 @@ class ErrorHandler(Cog):
         prepared_help_command = self.get_help_command(ctx)
 
         if isinstance(e, errors.MissingRequiredArgument):
-            await ctx.send(f"Missing required argument `{e.param.name}`.")
+            embed = self._get_error_embed("Missing required argument", e.param.name)
+            await ctx.send(embed=embed)
             await prepared_help_command
             self.bot.stats.incr("errors.missing_required_argument")
         elif isinstance(e, errors.TooManyArguments):
-            await ctx.send("Too many arguments provided.")
+            embed = self._get_error_embed("Too many arguments", str(e))
+            await ctx.send(embed=embed)
             await prepared_help_command
             self.bot.stats.incr("errors.too_many_arguments")
         elif isinstance(e, errors.BadArgument):
-            await ctx.send("Bad argument: Please double-check your input arguments and try again.\n")
+            embed = self._get_error_embed("Bad argument", str(e))
+            await ctx.send(embed=embed)
             await prepared_help_command
             self.bot.stats.incr("errors.bad_argument")
         elif isinstance(e, errors.BadUnionArgument):
-            await ctx.send(f"Bad argument: {e}\n```{e.errors[-1]}```")
+            embed = self._get_error_embed("Bad argument", f"{e}\n{e.errors[-1]}")
+            await ctx.send(embed=embed)
             self.bot.stats.incr("errors.bad_union_argument")
         elif isinstance(e, errors.ArgumentParsingError):
-            await ctx.send(f"Argument parsing error: {e}")
+            embed = self._get_error_embed("Argument parsing error", str(e))
+            await ctx.send(embed=embed)
             self.bot.stats.incr("errors.argument_parsing_error")
         else:
-            await ctx.send("Something about your input seems off. Check the arguments:")
+            embed = self._get_error_embed(
+                "Input error",
+                "Something about your input seems off. Check the arguments and try again."
+            )
+            await ctx.send(embed=embed)
             await prepared_help_command
             self.bot.stats.incr("errors.other_user_input_error")