Improve logging / add facility to log the cause of a connection failure

[TomKrauss]

In our project we have sporadic issues in production, where an exception could not be established due to some kind of protocol error. The following line in class Connection terminates a connection, when a protocol error is received swallowing the cause of the error and the (possibly not useful) stack trace, making it almost impossible to track down the root cause of the error:

http/pkgs/http2/lib/src/connection.dart

Line 167 in e37093f

_terminate(ErrorCode.CONNECT_ERROR, causedByTransportError: true);

Requested is an option to be able to add some kind of error logging to aid debugging connection failures.

There are more places in the Connection class where errors are swallowed (e.g. when running the pingHandler), which might be worth to be considered for better error diagnostics as well.