when using an update site URL with an error (an invalid/uncheckable signature in this case) the resulting error in the web ui is unhelpful and unreadable.

Steps to reproduce

• Start Jenkins (mvn jetty:run -pl war
• in the plugin manager set the update site URL to https://jenkins-updates.cloudbees.com/update-center/core-cm/update-center.json
• check for plugin updates

Expected results

• a human decipherable text image describing the error (e.g. "Signature verification failed in update site\nPath does not chain with any of the trust anchors"

Actual results

• A big red box with stack traces that flows off the screen that you can not understand what the issue is