Fix #1762

nix-store --export, nix-store --dump, and nix dump-path would previously
fail silently if writing the data out failed, because
 a) FdSink::write ignored exceptions, and
 b) the commands relied on FdSink's destructor, which ignores
    exceptions, to flush the data out.

This could cause rather opaque issues with installing nixos, because
nix-store --export would happily proceed even if it couldn't write its
data out (e.g. if nix-store --import on the other side of the pipe
failed).

This commit adds tests that expose these issues in the nix-store
commands, and fixes them for all three.

src/libutil/serialise.cc

@@ -67,7 +67,8 @@ void FdSink::write(const unsigned char * data, size_t len)
     try {
         writeFull(fd, data, len);
     } catch (SysError & e) {
-        _good = true;
+        _good = false;
+        throw;
     }
 }