[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.

docs/ext/commands/api.rst

@@ -296,6 +296,9 @@ Converters
 .. autoclass:: discord.ext.commands.InviteConverter
     :members:
 
+.. autoclass:: discord.ext.commands.GuildConverter
+    :members:
+
 .. autoclass:: discord.ext.commands.RoleConverter
     :members:
 
@@ -410,6 +413,9 @@ Exceptions
 .. autoexception:: discord.ext.commands.MemberNotFound
     :members:
 
+.. autoexception:: discord.ext.commands.GuildNotFound
+    :members:
+
 .. autoexception:: discord.ext.commands.UserNotFound
     :members: