[commands] Redesign extension exception flow.

Instead of raising a whole variety of exceptions, they are now wrapped
into ExtensionError derived classes.

* ExtensionAlreadyLoaded
	* Raised when an extension is already loaded in Bot.load_extension
* ExtensionNotLoaded
	* Raised when an extension is not loaded, e.g. Bot.unload_extension
* NoEntryPointError
	* Raised when an extension does not have a `setup` function.
* ExtensionFailed
	* Raised when an extension's `setup` function fails.
* ExtensionNotFound
	* Raised when an extension's module import fails.

discord/ext/commands/errors.py

@@ -34,7 +34,9 @@ __all__ = ['CommandError', 'MissingRequiredArgument', 'BadArgument',
            'MissingPermissions', 'BotMissingPermissions', 'ConversionError',
            'BadUnionArgument', 'ArgumentParsingError',
            'UnexpectedQuoteError', 'InvalidEndOfQuotedStringError',
-           'ExpectedClosingQuoteError', ]
+           'ExpectedClosingQuoteError', 'ExtensionError', 'ExtensionAlreadyLoaded',
+           'ExtensionNotLoaded', 'NoEntryPointError', 'ExtensionFailed',
+           'ExtensionNotFound' ]
 
 class CommandError(DiscordException):
     r"""The base exception type for all command related errors.
@@ -283,3 +285,80 @@ class ExpectedClosingQuoteError(ArgumentParsingError):
     def __init__(self, close_quote):
         self.close_quote = close_quote
         super().__init__('Expected closing {}.'.format(close_quote))
+
+class ExtensionError(DiscordException):
+    """Base exception for extension related errors.
+
+    This inherits from :exc:`~discord.DiscordException`.
+
+    Parameter
+    -----------
+    name: :class:`str`
+        The extension that had an error.
+    """
+    def __init__(self, message=None, *args, name):
+        self.name = name
+        message = message or 'Extension {!r} had an error.'.format(name)
+        # clean-up @everyone and @here mentions
+        m = message.replace('@everyone', '@\u200beveryone').replace('@here', '@\u200bhere')
+        super().__init__(m, *args)
+
+class ExtensionAlreadyLoaded(ExtensionError):
+    """An exception raised when an extension has already been loaded.
+
+    This inherits from :exc:`ExtensionError`
+    """
+    def __init__(self, name):
+        super().__init__('Extension {!r} is already loaded.'.format(name), name=name)
+
+class ExtensionNotLoaded(ExtensionError):
+    """An exception raised when an extension was not loaded.
+
+    This inherits from :exc:`ExtensionError`
+    """
+    def __init__(self, name):
+        super().__init__('Extension {!r} has not been loaded.'.format(name), name=name)
+
+class NoEntryPointError(ExtensionError):
+    """An exception raised when an extension does not have a ``setup`` entry point function.
+
+    This inherits from :exc:`ExtensionError`
+    """
+    def __init__(self, name):
+        super().__init__("Extension {!r} has no 'setup' function.".format(name), name=name)
+
+class ExtensionFailed(ExtensionError):
+    """An exception raised when an extension failed to load during execution of the ``setup`` entry point.
+
+    This inherits from :exc:`ExtensionError`
+
+    Attributes
+    -----------
+    name: :class:`str`
+        The extension that had the error.
+    original: :exc:`Exception`
+        The original exception that was raised. You can also get this via
+        the ``__cause__`` attribute.
+    """
+    def __init__(self, name, original):
+        self.original = original
+        fmt = 'Extension {0!r} raised an error: {1.__class__.__name__}: {1}'
+        super().__init__(fmt.format(name, original), name=name)
+
+class ExtensionNotFound(ExtensionError):
+    """An exception raised when an extension failed to be imported.
+
+    This inherits from :exc:`ExtensionError`
+
+    Attributes
+    -----------
+    name: :class:`str`
+        The extension that had the error.
+    original: :exc:`ImportError`
+        The original exception that was raised. You can also get this via
+        the ``__cause__`` attribute.
+    """
+    def __init__(self, name, original):
+        self.original = original
+        fmt = 'Extension {0!r} could not be loaded.'
+        super().__init__(fmt.format(name), name=name)