python: Try to be more helpful when given a string to encrypt().

* lang/python/helpers.c (_gpg_obj2gpgme_data_t): Extended error
message.
* lang/python/tests/t-encrypt.py: Test for "encode" in error message.

The motivation is to help the user when encrypting fails. I claim that
it is not obvious to not being able to encrypt a string directly. To
nudge the user into encoding it to bytes, the error message is a bit
extended.

• Signed-off-by: Tobias Mueller <muelli@cryptobitch.de>