feedback: add message to exception.

Author: zhumin8

oauth2_http/java/com/google/auth/oauth2/ImpersonatedCredentials.java

@@ -353,10 +353,10 @@ public byte[] sign(byte[] toSign) {
           transportFactory.create(),
           toSign,
           ImmutableMap.of("delegates", this.delegates));
-    } catch (IOException e) {
+    } catch (IOException ex) {
       // Throwing an IOException would be a breaking change, so wrap it here.
       // This should not happen for this credential type.
-      throw new IllegalStateException(e);
+      throw new IllegalStateException("Failed to sign: Error obtaining universe domain", ex);
     }
   }