Check for warnings in err stream too, to make javadoc task work on Java 17+

Discussed in dev list https://mail-archives.apache.org/mod_mbox/ant-dev/202106.mbox/%3c2e2ef67a-7d96-adfc-f050-efaccab7e6c2@apache.org%3e

WHATSNEW

@@ -27,6 +27,9 @@ Other changes:
  * Ant tries to avoid file name canonicalization when possible.
    Bugzilla Report 65499
 
+ * javadoc task will now look for warning messages in the STDERR stream too
+   when "failonwarning" is set to true to account for changes in JDK 17+
+
 Changes from Ant 1.10.10 TO Ant 1.10.11
 =======================================
 

src/main/org/apache/tools/ant/taskdefs/Javadoc.java

@@ -1918,7 +1918,7 @@ public class Javadoc extends Task {
                 throw new BuildException("Javadoc returned " + ret,
                                          getLocation());
             }
-            if (out.sawWarnings() && failOnWarning) {
+            if (failOnWarning && (out.sawWarnings() || err.sawWarnings())) {
                 throw new BuildException("Javadoc issued warnings.",
                                          getLocation());
             }
@@ -2623,7 +2623,7 @@ public class Javadoc extends Task {
 
         @Override
         protected void processLine(final String line, final int messageLevel) {
-            if (line.contains("warning")) {
+            if (line.matches("(\\d) warning[s]?$")) {
                 sawWarnings = true;
             }
             if (messageLevel == Project.MSG_INFO