[commands] Add discord.Guild converter and GuildNotFound error

* Add discord.Guild converter and GuildNotFound error

* note for lack of disambiguation in Guilds with duplicate names, and removed the possibility of returning None

* edited converter to use `utils.get` over `utils.find` and docs edited with Converter and Exception.

discord/ext/commands/errors.py

@@ -45,6 +45,7 @@ __all__ = (
     'NotOwner',
     'MessageNotFound',
     'MemberNotFound',
+    'GuildNotFound',
     'UserNotFound',
     'ChannelNotFound',
     'ChannelNotReadable',
@@ -230,6 +231,22 @@ class MemberNotFound(BadArgument):
         self.argument = argument
         super().__init__('Member "{}" not found.'.format(argument))
 
+class GuildNotFound(BadArgument):
+    """Exception raised when the guild provided was not found in the bot's cache.
+
+    This inherits from :exc:`BadArgument`
+
+    .. versionadded:: 1.7
+
+    Attributes
+    -----------
+    argument: :class:`str`
+        The guild supplied by the called that was not found
+    """
+    def __init__(self, argument):
+        self.argument = argument
+        super().__init__('Guild "{}" not found.'.format(argument))
+
 class UserNotFound(BadArgument):
     """Exception raised when the user provided was not found in the bot's
     cache.