Also sending the exception to the log, which is where plugin developers expect to see it.
(This is also necessary for the exception to be visible say from configRoundtrip in a functional test.
Otherwise you just see a FailingHttpStatusCodeException due to the 400 error code, with no details.)
Might actually suffice to just throw a ServletException wrapping the exception up the chain,
letting Stapler or the container handle it, but this change is more conservative.