Improvements to PuTTYgen error reporting:

 - will now display a reason when it fails to load a key
 - uses existing error return from native keys
 - import.c had a lot of error descriptions which weren't going anywhere;
   since the strings are probably taking up space in the binary, we
   may as well use them

git-svn-id: svn://svn.tartarus.org/sgt/putty@5408 cda61777-01e9-0310-a592-d414129be87e