Fix bug when displaying example errors

We were trying to concatenate a list of nets, but "concatenate"
expects a list of strings.

M2/Macaulay2/m2/installPackage.m2

@@ -771,7 +771,7 @@ installPackage Package := opts -> pkg -> (
 	generateExampleResults(pkg, rawDocumentationCache, exampleDir, exampleOutputDir, verboseLog, pkgopts, opts);
 
 	if 0 < numExampleErrors then verboseLog concatenate apply(readDirectory exampleOutputDir,
-	    file -> if match("\\.errors$", file) then stack {
+	    file -> if match("\\.errors$", file) then toString stack {
 		file, concatenate(width file : "*"), getErrors(exampleOutputDir | file)});
 
 	if not opts.IgnoreExampleErrors then checkForErrors pkg;