more descriptive error messages for transport connection errors

[shanejonas]

Describe the bug
As a user having just code: 7979 is not enough to debug some issues.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://playground.open-rpc.org/ and hit the inspector play button against a non-existant server
2. See error

Expected behavior
I should see a more descriptive error returned

Screenshots

[bradjones1]

Will including the response in the error object for HTTP transport help with this/solve this? #307