* The determination of the root set should be made by the privileged

process, so forward the operation.
* Spam the user about GC misconfigurations (NIX-71).
* findRoots: skip all roots that are unreadable - the warnings with
  which we spam the user should be enough.

src/libutil/util.cc

@@ -35,6 +35,7 @@ Error & Error::addPrefix(const format & f)
 
 SysError::SysError(const format & f)
     : Error(format("%1%: %2%") % f.str() % strerror(errno))
+    , errNo(errno)
 {
 }