Update error messages for non-async servletfilters .

This was a pmr /case TS000806721 and it was agreed to post it as a RFE.

I am using latest liberty profile 17.0.0.x (tried .2 and .3) with the
shipped JDK8 from wasdev. I also tried openliberty with the same result.

If you use Servlet's asyncresponse and use a Servlet-Filter in web.xml,
Liberty will throw (correctly) an error SC for those servlets. However,
liberty will *only* give a SC 500 with no further explanation:

Caused by: javax.servlet.ServletException: Filtered request failed.
at org.apache.shiro.web.servlet.AbstractShiroFilter.doFilterInternal
(AbstractShiroFilter.java:384)
at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter
(OncePerRequestFilter.java:125)
at com.ibm.ws.webcontainer.filter.FilterInstanceWrapper.doFilter
(FilterInstanceWrapper.java:207)
... 1 more
Caused by: java.lang.NullPointerException
at org.apache.cxf.jaxrs.impl.AsyncResponseImpl.initContinuation
(AsyncResponseImpl.java:357)


The stack trace does not give a good hint of what is misconfigured.
Other products (like wildfly) give a hint that the filter is not async-
enabled
and will refuse to run the servlet at all.

Proposed solution:
1.) Give a hint that a non-async-enabled Filter is being used for async
servlets
2.) Give a hint that a possible workaround is to add supported>true to web.xml, but only if the
user/developer is sure, that the Servlet is indeed async capable.

Please consider this and also include this in WDT for Eclipse.

Thank you very much!


Reproducible: always.


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