Ticket #3406: sftpfs: fix -31 SFTP Protocol Error when transferring file via SFTP Link.
Initial commit: extend error message with sftp session error.
When LIBSSH2_ERROR_SFTP_PROTOCOL (-31) occurs extend error message
with sftp last error for better failure tracking.
With #3269 patch further sftp protocol error message now looks like
SFTP Protocol Error 3 (-31).
Currently I have no nice solution for print relevant called libssh2 function too
I don't want pollute _to_gliberror() with contant strings all over.
sftp session error 3 means (file) permission denied.
From libssh2-1.5.0/include/libssh2_sftp.h
Signed-off-by: Andreas Mohr <and@gmx.li>
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>