Warn user if .p12 support is not available because of missing crypto libraries

[ddoskind]

If you try to create ServiceAccountCredentials without having PyOpenSSL installed, a UnicodeDecodeError is raised:

>>> from oauth2client import service_account
>>> service_account.ServiceAccountCredentials.from_p12_keyfile(CLIENT_ID, PATH_TO_P12_KEY_FILE)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "lib/python2.7/site-packages/oauth2client/service_account.py", line 245, in from_p12_keyfile
    private_key_password)
  File "lib/python2.7/site-packages/oauth2client/_pure_python_crypt.py", line 167, in from_string
    key = _from_bytes(key)  # pem expects str in Py3
  File "lib/python2.7/site-packages/oauth2client/_helpers.py", line 87, in _from_bytes
    if isinstance(value, six.binary_type) else value)
  File "lib/python2.7/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeDecodeError: 'utf8' codec can't decode byte 0x82 in position 1: invalid start byte

It would be better to use the existing (but unused, from what I can tell) _PKCS12_ERROR string inside service_account.py to let the user know what's going on.

[dhermes]

Thanks for filing! I thought I caught this before it got parsed, though it seems I didn't check all three crypto implementations.