Enhance error reporting on failed OS cal

It would be helpful to have better diagnostics on OS calls like file open that fail, both more precise error messages, but also setting Tcl's errorCode variable. Compare:

  [powerbook:/tmp] roseman% tclkit
  % open sitedata.db
  couldn't open "sitedata.db": permission denied
  % set errorCode
  POSIX EACCES {permission denied}
  % mk::file open db sitedata.db
  file open failed
  % set errorCode
  NONE
  %

(man Tcl_SetErrorCode)