Complete todo in SelfSignedCertificate (netty#9720)

Motivation:

Easier to debug SelfSignedCertificate failures.

Modifications:

Add first throwable as suppressed to thrown exception.

Result:

Less technical debt.

handler/src/main/java/io/netty/handler/ssl/util/SelfSignedCertificate.java

@@ -21,6 +21,7 @@
 import io.netty.handler.codec.base64.Base64;
 import io.netty.util.CharsetUtil;
 import io.netty.util.internal.SystemPropertyUtil;
+import io.netty.util.internal.ThrowableUtil;
 import io.netty.util.internal.logging.InternalLogger;
 import io.netty.util.internal.logging.InternalLoggerFactory;
 
@@ -162,10 +163,11 @@ public SelfSignedCertificate(String fqdn, SecureRandom random, int bits, Date no
                 paths = BouncyCastleSelfSignedCertGenerator.generate(fqdn, keypair, random, notBefore, notAfter);
             } catch (Throwable t2) {
                 logger.debug("Failed to generate a self-signed X.509 certificate using Bouncy Castle:", t2);
-                throw new CertificateException(
+                final CertificateException certificateException = new CertificateException(
                         "No provider succeeded to generate a self-signed certificate. " +
                                 "See debug log for the root cause.", t2);
-                // TODO: consider using Java 7 addSuppressed to append t
+                ThrowableUtil.addSuppressed(certificateException, t);
+                throw certificateException;
             }
         }