Fix NameError in ClientApplication exception handler

[chlowell]

To reproduce the error:

from msal import PublicClientApplication

class Client:
    def get(*args, **kwargs):
        raise Exception('oops')

PublicClientApplication('...', http_client=Client())

Traceback (most recent call last):
  File "repro.py", line 8, in <module>
    PublicClientApplication("...", http_client=Client())
  File "...lib\site-packages\msal\application.py", line 1296, in __init__
    super(PublicClientApplication, self).__init__(
  File "...lib\site-packages\msal\application.py", line 302, in __init__
    if validate_authority and region:
NameError: name 'region' is not defined

I assume the keyword argument azure_region is intended.

[rayluo]

Argh, our last-minute change from region to azure_region left this one behind. My bad.

Nice catch! Thanks!