Reset the response before sending a 500 after an exception

[michaelJustin]

Closes #564.

When a Web Component or Web Filter raised after it had started its response, the 500 kept what the handler had already set:

• a stale Content-Length. Indy recalculates it only when it is -1, so the error page arrived truncated, or the client waited for bytes that never came;
• its Content-Type, so the HTML error page went out labelled application/json;
• its custom headers;
• for a filter, its partial body.

Changes

• New unit djErrorResponse, shared by the three places that build a 500:
  • ResetResponse calls Indy's Clear, keeping the server name, the keep-alive setting and the cookies (Indy stores a new session's cookie there). It also drops ContentText and ContentStream.
  • SetErrorResponse resets and writes the framework's 500 page. If the headers were already sent, it drops pending content and closes the connection instead of appending to a partial body.
• TdjWebComponentHandler.Handle builds the error response for every exception from the dispatch: lazy Start, the filter chain and the component. Filters are covered too, and the development-mode page still names the component. InvokeService now only logs and re-raises.
• TdjWebComponentContextHandler.DoHandle resets the response before calling the ErrorHandler. If the ErrorHandler raises, its partial output is replaced with the default page. Once the headers have been sent, the ErrorHandler is skipped.
• TdjHTTPConnector.OnCommand uses the same SetErrorResponse, replacing its own copy of the page.
• Delphi test project: Unittests.dpr and .dproj list the new unit.

Tests

Three new tests in TAPIConfigTests:

• a component, and separately a filter, that set Content-Type, a too-short Content-Length, a custom header and part of a body before raising;
• an ErrorHandler that sets only a body.

All three fail on the old code (checked by running them against the unchanged sources), and pass now.

• FPC (run-fpc.cmd): 201 tests, 0 errors, 0 failures; heap trace at the clean baseline (6 blocks / 744 bytes).
• Delphi 2009 (run-delphi.cmd): OK, 201 tests.
• djErrorResponse also compiles with DARAJA_PROJECT_STAGE_DEVELOPMENT on both compilers. The madExcept and JCL stack-trace variants are not built here.

The development-mode page changes slightly. It now shows the exception class and message, the component name and the stack trace in a single layout, and it's also used for filter exceptions.

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