Add LoginFailure exception for a clearer failure in Client.login

The older HTTPException is not exactly the clearest thing for people
who are new to programming or HTTP exceptions in general.

discord/client.py

@@ -810,7 +810,7 @@ class Client(object):
 
         After this function is called, :attr:`is_logged_in` returns True if no
         errors occur. If an error occurs during the login process, then
-        :exc:`HTTPException` is raised.
+        :exc:`LoginFailure` or :exc:`HTTPException` is raised.
 
         This function raises :exc:`GatewayNotFound` if it was unavailable to connect
         to a websocket gateway.
@@ -826,7 +826,10 @@ class Client(object):
 
         r = requests.post(endpoints.LOGIN, json=payload)
         log.debug(request_logging_format.format(response=r))
-        utils._verify_successful_response(r)
+        if r.status_code == 400:
+            raise LoginFailure('Improper credentials have been passed.')
+        elif r.status_code != 200:
+            raise HTTPException(r)
 
         log.info('logging in returned status code {}'.format(r.status_code))
         self.email = email

discord/errors.py

@@ -88,3 +88,10 @@ class InvalidArgument(ClientException):
     :exc:`DiscordException`.
     """
     pass
+
+class LoginFailure(ClientException):
+    """Exception that's thrown when the :meth:`Client.login` function
+    fails to log you in from improper credentials or some other misc.
+    failure.
+    """
+    pass

docs/api.rst

@@ -311,6 +311,8 @@ The following exceptions are thrown by the library.
 
 .. autoexception:: ClientException
 
+.. autoexception:: LoginFailure
+
 .. autoexception:: HTTPException
     :members: