fix Lwt_unix.wrap_syscall
fix Lwt_unix.wrap_syscall

Ignore-this: 6f807da1067a201b090dc0142ad6e5bf

When "lwt blocking = ... in" yielded, the exception raised by "action
()" escaped the try ... with.

darcs-hash:20120406161031-c41ad-cd33708abf4e492a036f0aad10080719cee543e8

gfxmonk/lwt,fdopen/lwt,fdopen/lwt,janestreet/lwt-async,rgrinberg/lwt,mirage/lwt,mfp/lwt,ocsigen/lwt,gfxmonk/lwt,rgrinberg/lwt,rgrinberg/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,mirage/lwt,ocsigen/lwt,c-cube/lwt,rgrinberg/lwt,c-cube/lwt,rneswold/lwt,janestreet/lwt-async,mfp/lwt,rneswold/lwt