If the file _repo.tar.gz exists and is non-writable, "darcs dist" prints

  gzip: compressed data not written to a terminal. Use -f to force compression.
  For help, type: gzip -h
  Created dist as _repo.tar.gz

and returns with success.
But the dist tar file has *not* been created (overwritten).

Darcs should catch the error and fail.

I suspect this has something to do with the exec function in Exec.lhs.
When it redirects to a file and the open fails, it seems to continue
with the redirection unchanged (the unix part of the code).

The windows part of exec (using system) complains with

  /bin/sh: line 1: _repo.tar.gz: Permission denied
  Created dist as _repo.tar.gz

and still exits with success.

For what it's worth, the new current version under Unix exits with code 3 and
complains.  Still cryptic, though.

Failed to execute external command: gzip -c
Lowlevel error: open_write: Permission denied
Redirects: (File "/private/tmp/foo/darcsmdVEbo",File
"/private/tmp/foo/foo.tar.gz",AsIs)

When I tried this today with "darcs dist" using darcs-unstable, I got a rather
clear error message after making the target file file not writable:

darcs: /home/mark/Haskell/tmp/tmp.tar.gz: openBinaryFile: permission denied
(Permission denied)

I tried again with darcs 1.0.9 and got a message like the ones reported below,
so I'm considering this resolved-in-unstable.