Currently Tomcat throws an unchecked IllegalStateException.
The API presumably expects the Response to be returned and throw a
checked IOException on writing.
But Tomcat justifiably want to recycle so the proposal is some new
checked Exception in the API.
Without this there is no proper way to write the code as the
error/timeout handler cannot be atomically synchronized with its causal
event in a Thread race.
If accepted, presuamably similar API change for getRequest.