MatrixHttpApi._send() can call sleep() with undefined _waittime_

If any exception other than KeyError is thrown, sleep will be called in

matrix-python-sdk/matrix_client/api.py

Line 679 in 6a35a9c

sleep(waittime)

with its parameter waittime undefined. Introduced in commit 3c3e910

[non-Jedi]

Path forward here is to make sure waittime has a value even if exception not caught. It isn't to catch all exceptions (for anyone trying to fix this). sleep should also be moved out of the finally block. If some other exception is raised, we don't want to be sleeping before we raise it.