Show a backtrace when tool/extlibs.rb fails

I'd like to retry this kind of error, but showing no backtrace is hard
to deal with.
https://github.com/ruby/ruby/runs/2644908002

tool/extlibs.rb

@@ -243,7 +243,7 @@ class ExtLibs
           begin
             extracted = do_command(mode, dest, url, cache_dir, chksums)
           rescue => e
-            warn e.inspect
+            warn e.full_message
             success = false
           end
           url = chksums = nil