Add message when decoding PKCS12 file fails.

Add message when decoding PKCS12 file fails.

Currently we never display the OpenSSL error stack when decoding a
PCKS12 file fails. With LibreSSL defaulting to RC2-40-CBC, the failure
might not be a wrong password but can actually be an unsupported encoding,
seeing the error stack is really helpful (example from OpenSSL 3.0):

error:0308010C:digital envelope routines:inner_evp_generic_fetch:
  unsupported:crypto/evp/evp_fetch.c:346:Global default library
  context, Algorithm (RC2-40-CBC : 0), Properties ()

to pinpoint the issue

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211019183127.614175-17-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23017.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>