better error handling for backends

basis/io/backend/unix/unix.factor

@@ -51,6 +51,7 @@ M: unix (stream-seek) ( n seek-type stream -- )
         { io:seek-absolute [ SEEK_SET ] }
         { io:seek-relative [ SEEK_CUR ] }
         { io:seek-end [ SEEK_END ] }
+        [ io:bad-seek-type ]
     } case
     [ handle>> fd>> swap ] dip lseek io-error ;
 

basis/io/backend/windows/nt/nt.factor

@@ -92,6 +92,7 @@ M: winnt (stream-seek) ( n seek-type stream -- )
         { seek-absolute [ handle>> (>>ptr) ] }
         { seek-relative [ handle>> [ + ] change-ptr drop ] }
         { seek-end [ handle>> [ handle>> handle>file-size + ] keep (>>ptr) ] }
+        [ bad-seek-type ]
     } case ;
 
 : file-error? ( n -- eof? )