unshareFilesystem: Do not assume caller

src/libstore/filetransfer.cc

@@ -580,7 +580,12 @@ struct curlFileTransfer : public FileTransfer
         #endif
 
         #if __linux__
-        unshareFilesystem();
+        try {
+            unshareFilesystem();
+        } catch (nix::Error & e) {
+            e.addTrace({}, "in download thread");
+            throw;
+        }
         #endif
 
         std::map<CURL *, std::shared_ptr<TransferItem>> items;

src/libutil/linux/namespaces.cc

@@ -140,7 +140,7 @@ void restoreMountNamespace()
 void unshareFilesystem()
 {
     if (unshare(CLONE_FS) != 0 && errno != EPERM)
-        throw SysError("unsharing filesystem state in download thread");
+        throw SysError("unsharing filesystem state");
 }
 
 }