Nicer error msg for missing a library.

crypt.py

# Import third party libs
try:
    from M2Crypto import RSA
    from Crypto.Cipher import AES
except ImportError:
    # No need for crypt in local mode, but give a nicer message 
    class CryptoErrorClass():
        def __init__(self, necessary_lib):
            self._lib = necessary_lib
        def __getattr__(self, attr):
            log.error('Missing %s library, needed for remote master communication.', self._lib)
            raise Exception('Missing {0} library.'.format(self._lib))
    RSA = CryptoErrorClass('M2Crypto')
    AES = CryptoErrorClass('Crypto')
            
...
    gen = RSA.gen_key(...)
... -> Exception: You need to install m2crypto.