Fix AsyncContext to be completed on AsyncListener.onError()

[injae-kim]

Closes gh-31541

Motivation

According to https://bz.apache.org/bugzilla/show_bug.cgi?id=66875#c11,
our StandardServletAsyncWebRequest.onError implementation should call asyncContext.complete()
if that context is still non-null.
Let's sort this out for 6.1.3 and backport it to 6.0.16 and 5.3.32 from there.

• On Double error response handling on suspended methods #31541 (comment), we found that our StandardServletAsyncWebRequest.onError implementation should call asyncContext.complete() if that context is still non-null.

Tomcat's error handling triggers the onError event.
...
Spring and/or the application should be calling complete() or dispatch() as a result of the onError() event.

• See bug report for the details

Modification

• Fix AsyncContext to be completed on StandardServletAsyncWebRequest.onError()

Result

• Closes Double error response handling on suspended methods #31541

[rstoyanchev]

As per #31541 (comment), this is not the fix we want. Thanks for preparing the pull request in any case!