crypto: use error_abort for unexpected failures

crypto: use error_abort for unexpected failures

This improves the error diagnosis from the unit test when a cipher
is unexpected not available from

ERROR:../tests/unit/test-crypto-cipher.c:683:test_cipher: assertion failed: (err == NULL)
Bail out! ERROR:../tests/unit/test-crypto-cipher.c:683:test_cipher: assertion failed: (err == NULL)
Aborted (core dumped)

to

Unexpected error in qcrypto_cipher_ctx_new() at ../crypto/cipher-gcrypt.c.inc:262:
./build//tests/unit/test-crypto-cipher: Cannot initialize cipher: Invalid cipher algorithm
Aborted (core dumped)

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>