Add error code for encryption failure in pgcrypto

Add error code for encryption failure in pgcrypto

PXE_DECRYPT_FAILED exists already for decryption errors, and an
equivalent for encryption did not exist.  There is one code path that
deals with such failures for OpenSSL but it used PXE_ERR_GENERIC, which
was inconsistent.  This switches this code path to use the new error
PXE_ENCRYPT_FAILED instead of PXE_ERR_GENERIC, making the code used for
encryption more consistent with the decryption.

Author: Daniel Gustafsson
Discussion: https://postgr.es/m/03049139-CB7A-436E-B71B-42696D3E2EF7@yesql.se