## Motivation
A gRPC server may send a HTTP/2 GOAWAY frame with NO_ERROR status to gracefully shutdown a connection. This appears to Tonic users as a `tonic::Status` with `Code::Internal` and the message set to `h2 protocol error: protocol error: not a result of an error`.
The only way to currently detect this case and differentiate it from other internal errors (e.g., an application-level internal error) is to match on the message. A client may want to differentiate these cases because it may only want to alert on the application-level internal error and not on the transient transport-level issue. (Indeed, this is the use case for which I'm envisioning using this change.)
Matching on a message is not as robust, however, as matching on an `h2::Error` and its reason code. (The message could change for example if a future version of Tonic decided to vary the message. This would break any users that matched on the previous version of the message.)
## Solution
Store the `h2::Error` used when creating a `tonic::Status` from a `h2::Error` and provide it as the `source` for purposes of `std::error::Error`. This will allow users to downcast it and match on the original `h2::Reason`.
