bare except: clause is bad

At least under gunicorn, SystemExit can be raised anywhere.

2014-06-20 11:40:39,998 bkr.server.xmlrpccontroller ERROR Error handling XML-RPC method
Traceback (most recent call last):
  File "Server/bkr/server/xmlrpccontroller.py", line 53, in RPC2
    response = self.process_rpc(method,params)
  File "Server/bkr/server/xmlrpccontroller.py", line 42, in process_rpc
    response = obj(*params)
  File "Server/bkr/server/authentication.py", line 76, in login_password
    if not user.check_password(password):
  File "Server/bkr/server/model/identity.py", line 241, in check_password
    raw_password, self._password)
  File "/usr/lib/python2.6/site-packages/passlib/context.py", line 2592, in verify_and_update
    if not record.verify(secret, hash, **kwds):
  File "/usr/lib/python2.6/site-packages/passlib/utils/handlers.py", line 571, in verify
    return consteq(self._calc_checksum(secret), chk)
  File "/usr/lib/python2.6/site-packages/passlib/handlers/pbkdf2.py", line 84, in _calc_checksum
    return pbkdf2(secret, self.salt, self.rounds, self.checksum_size, self._prf)
  File "/usr/lib/python2.6/site-packages/passlib/utils/pbkdf2.py", line 411, in pbkdf2
    return join_bytes(gen())[:keylen]
  File "/usr/lib/python2.6/site-packages/passlib/utils/pbkdf2.py", line 408, in gen
    digest = prf_func(secret, digest)
  File "/usr/lib/python2.6/site-packages/passlib/utils/pbkdf2.py", line 174, in prf
    return hmac_const(key, msg, digest)
  File "/usr/lib64/python2.6/site-packages/M2Crypto/EVP.py", line 98, in hmac
    return m2.hmac(key, data, md())
  File "/usr/lib/python2.6/site-packages/gunicorn/workers/base.py", line 137, in handle_exit
    sys.exit(0)
SystemExit: 0

Change-Id: I58dd93798bdd907c4f2c8a5beda89a7caa623ab6