HttpException stacktrace is lost when ClientException is thrown

When a `ClientException` is thrown [from here in `io_client.dart`](https://github.com/dart-lang/http/blob/7774121a406fedb0502fce7cc224476c1ac78469/lib/src/io_client.dart#L65), it does not preserve the original HttpException's stacktrace (because of [this still open issue](https://github.com/dart-lang/sdk/issues/30741) there is no way to preserve the original stacktrace if not rethrowing the original exception).

This makes it almost impossible to debug issues such as [this one](https://github.com/dart-lang/sdk/issues/37695) when the exceptions are being reported via crash analytics and are not easily reproducible under a debugger.

The throw path is lib/src/io_client.dart around line 65; start by reading how ClientException is created and review Dart SDK issue 30741. Done means the original HttpException stack trace is available through ClientException for crash reporting, with the affected behavior covered by a regression test.

Written by the indexing model from the issue text.