Add stderr to error message in GClone spec searcher

This fixes a bug in the `GClone` spec searcher that was adding the
contents of stdout to the error message raised when cloning the git
repository fails. Now, the contents of stderr are added to the error
message.

Author: beechtom

lib/puppetfile-resolver/spec_searchers/git/gclone.rb

@@ -57,7 +57,7 @@ def self.clone_and_read_file(url, ref, file, config)
             clone_cmd.push("--branch=#{ref}") if ref != 'HEAD'
             clone_cmd.push(url, dir)
             out, err_out, process = ::PuppetfileResolver::Util.run_command(clone_cmd)
-            err_msg += out
+            err_msg += err_out
             raise err_msg unless process.success?
             Dir.chdir(dir) do
               content, err_out, process = ::PuppetfileResolver::Util.run_command(['git', 'show', "#{ref}:#{file}"])