Fix 'error 9 while decompressing xz file'

Once we've started writing data to a Sink, we can't restart a download
request, because then we end up writing duplicate data to the
Sink. Therefore we shouldn't handle retries in Downloader but at a
higher level (in particular, in copyStorePath()).

Fixes #2952.

src/libutil/types.hh

@@ -109,6 +109,8 @@ public:
     const string & msg() const { return err; }
     const string & prefix() const { return prefix_; }
     BaseError & addPrefix(const FormatOrString & fs);
+
+    virtual bool isTransient() { return false; }
 };
 
 #define MakeError(newClass, superClass) \