Gracefully handle unknown host exceptions during OpenID discovery

We don't need these errors in our application server log.  If the
client has given us a hostname we can't resolve, trap the error
and return an invalid discovery JSON payload to them instead.  The
user will see a message explaining its an unsupported/invalid URL
and can try again.

Signed-off-by: Shawn O. Pearce <sop@google.com>

appjar/src/main/java/com/google/gerrit/server/OpenIdLoginServlet.java

diff --git a/appjar/src/main/java/com/google/gerrit/server/OpenIdLoginServlet.java b/appjar/src/main/java/com/google/gerrit/server/OpenIdLoginServlet.java
index 91f1bd2..d8088d8 100644
--- a/appjar/src/main/java/com/google/gerrit/server/OpenIdLoginServlet.java
+++ b/appjar/src/main/java/com/google/gerrit/server/OpenIdLoginServlet.java
@@ -50,6 +50,7 @@
 import java.io.IOException;
 import java.io.OutputStream;
 import java.io.StringWriter;
+import java.net.UnknownHostException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Enumeration;
@@ -152,7 +153,16 @@
       return;
     }
 
-    final OpenIdUser user = relyingParty.discover(req);
+    final OpenIdUser user;
+    try {
+      user = relyingParty.discover(req);
+    } catch (UnknownHostException u) {
+      // The remote host described in the OpenID doesn't exist, so we
+      // can't try to perform discovery against it.
+      //
+      callback(req, rsp, SignInResult.CANCEL);
+      return;
+    }
     if (user == null) {
       // User isn't known, no provider is known.
       //