[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.

docs/ext/commands/api.rst

@@ -285,6 +285,25 @@ Exceptions
 .. autoexception:: discord.ext.commands.BotMissingPermissions
     :members:
 
+.. autoexception:: discord.ext.commands.ExtensionError
+    :members:
+
+.. autoexception:: discord.ext.commands.ExtensionAlreadyLoaded
+    :members:
+
+.. autoexception:: discord.ext.commands.ExtensionNotLoaded
+    :members:
+
+.. autoexception:: discord.ext.commands.NoEntryPointError
+    :members:
+
+.. autoexception:: discord.ext.commands.ExtensionFailed
+    :members:
+
+.. autoexception:: discord.ext.commands.ExtensionNotFound
+    :members:
+
+
 Exception Hierarchy
 +++++++++++++++++++++
 
@@ -311,3 +330,9 @@ Exception Hierarchy
             - :exc:`~.commands.DisabledCommand`
             - :exc:`~.commands.CommandInvokeError`
             - :exc:`~.commands.CommandOnCooldown`
+        - :exc:`~.commands.ExtensionError`
+            - :exc:`~.commands.ExtensionAlreadyLoaded`
+            - :exc:`~.commands.ExtensionNotLoaded`
+            - :exc:`~.commands.NoEntryPointError`
+            - :exc:`~.commands.ExtensionFailed`
+            - :exc:`~.commands.ExtensionNotFound`