mbedtls: improve error reporting in tls verify callback

mbedtls: improve error reporting in tls verify callback

Instead of just printing the contents of the flags variable, try to
convert it to a human-readable error string and print that instead.

This will for example print "The certificate is signed with an
unacceptable key (eg bad curve, RSA too short).", instead of
"flags=10000".

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1462306478-21059-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11594
Signed-off-by: Gert Doering <gert@greenie.muc.de>