Report AsyncFile handler failures to the context

[jnbdz]

Fixes #5343

Motivation

When the data handler or the end handler of an AsyncFile read stream throws, nothing happens: the InboundBuffer catches the throwable in handleEvent and, since AsyncFileImpl never registers an exception handler on its queue, the failure is silently dropped. Neither the stream exceptionHandler nor the context ever sees it (the issue's reproducer prints nothing).

As discussed on the issue, the correct behaviour is to report such failures to the context, like event-bus consumers do.

Changes

• AsyncFileImpl.handleBuffer / handleEnd now dispatch the handler through the context (ContextInternal.dispatch), so an exception thrown by the handler is reported via Context.reportException (context exception handler → Vertx.exceptionHandler → "Unhandled exception" log). The stream exceptionHandler keeps its I/O failure semantic and is not invoked for a handler failure; reading is not interrupted and the end handler is still called.
• Tests: FileSystemTest.testReadStreamHandlerExceptionReportedToContext / testReadStreamEndHandlerExceptionReportedToContext (fail without the change).

Note: NettyFileUpload (multipart uploads) has the same silent-swallow pattern with its InboundBuffer; left out of this PR since the issue is about AsyncFile, happy to address it here or separately if wanted.