send: better error message after find_mount_root fails

[kdave]

Copy todo item from wiki: Better error message in case of error when sending. Especially "ERROR: find_mount_root failed on %s:" is not of any help.

file: cmds/send.c, find_mount_root fails called from cmd_send

[realwakka]

I've checked find_mount_root() code. It seems that if there is an error (ret<0), ret should be comes from strdup or realpath longest_match. longest_match is from mount entry. I think that ENOMEM is the only error can be occur. Are there any other errors to print?