fix(mint): return errors for non-200 responses

[arrangabriel]

process_response({:status, ...}) now emits an RPCError into the stream when the status is not 200,
instead of only recording it, with the same status and message as GRPC.Client.Adapters.Gun.await/2.

Mint delivers :status ahead of :headers and :data, so the error is the first entry in the response
list: unary calls find it through check_for_error/1 before reaching Keyword.fetch!(responses, :ok), and
streaming calls find it as the first element of the stream. Both now return {:error, %GRPC.RPCError{}} rather
than raising.

The test injects a HEADERS frame carrying :status: 502 and asserts the error arrives ahead of the headers
that follow.

Fixes #596

[arrangabriel]

That failing test is the same one that has been flaky before @sleipnir