fix command error handling

so, i had a fundamental misunderstanding about how command errors work.
turns out the help command runs the checks for each function.
This ended up sending one "permission denied" message for each command in the Emotes cog,
*each time help was run*.

To fix this, __local_check needs to not have extra side effects.
I create one new exception, MissingManageServerEmojis, and raise it in case the user or bot is missing permissions.
I re-use NoPrivateMessages, since that's already a d.py exception.

Then I catch the errors in on_command_error, and send them to the user.

cogs/emote.py

@@ -48,26 +48,25 @@ class Emotes:
 
 	async def __local_check(self, context):
 		if not context.guild:
-			await context.send(
+			raise commands.NoPrivateMessage
-				f'{utils.SUCCESS_EMOTES[False]} Sorry, this command may only be used in a server.')
 			return False
 
 		if (
 			not context.author.guild_permissions.manage_emojis
 			or not context.guild.me.guild_permissions.manage_emojis
 		):
-			await context.send(
+			raise errors.MissingManageEmojisPermission
-				f'{utils.SUCCESS_EMOTES[False]} '
-				"Sorry, you don't have enough permissions to run this command. "
-				'You and I both need the Manage Emojis permission.')
-			return False
 
 		return True
 
 	async def on_command_error(self, context, error):
-		if isinstance(error, errors.EmoteManagerError):
+		if isinstance(error, (errors.EmoteManagerError, errors.MissingManageEmojisPermission)):
 			await context.send(str(error))
 
+		if isinstance(error, commands.NoPrivateMessage):
+			await context.send(
+				f'{utils.SUCCESS_EMOTES[False]} Sorry, this command may only be used in a server.')
+
 	@commands.command()
 	async def add(self, context, *args):
 		"""Add a new emote to this server.

utils/errors.py

@@ -1,44 +1,49 @@
 #!/usr/bin/env python3
 # encoding: utf-8
 
-from discord.ext.commands import CommandError
+from discord.ext import commands
+
+import utils
 
 
-class EmoteManagerError(CommandError):
+class MissingManageEmojisPermission(commands.MissingPermissions):
+	"""The invoker or the bot doesn't have permissions to manage server emojis."""
+
+	def __init__(self):
+		super(Exception, self).__init__(
+			f'{utils.SUCCESS_EMOTES[False]} '
+			"Sorry, you don't have enough permissions to run this command. "
+			'You and I both need the Manage Emojis permission.')
+
+class EmoteManagerError(commands.CommandError):
 	"""Generic error with the bot. This can be used to catch all bot errors."""
 	pass
 
-
 class HTTPException(EmoteManagerError):
 	"""The server did not respond with an OK status code."""
 	def __init__(self, status):
 		super().__init__(f'URL error: server returned error code {status}')
 
-
 class EmoteNotFoundError(EmoteManagerError):
 	"""An emote with that name was not found"""
 	def __init__(self, name):
 		super().__init__(f'An emote called `{name}` does not exist in this server.')
 
-
 class InvalidImageError(EmoteManagerError):
 	"""The image is not a GIF, PNG, or JPG"""
 	def __init__(self):
 		super().__init__('The image supplied was not a GIF, PNG, or JPG.')
 
-
 class NoMoreSlotsError(EmoteManagerError):
 	"""Raised in case all slots of a particular type (static/animated) are full"""
 	def __init__(self):
 		super().__init__('No more backend slots available.')
 
-
 class PermissionDeniedError(EmoteManagerError):
 	"""Raised when a user tries to modify an emote without the Manage Emojis permission"""
 	def __init__(self, name):
 		super().__init__(f"You're not authorized to modify `{name}`.")
 
-
 class DiscordError(Exception):
 	"""Usually raised when the client cache is being baka"""
 	def __init__(self):