This changes the error code returned by the RPC server in certain situations:

- handler panic: code -32603
- result marshaling error: code -32603
- attempt to subscribe via HTTP: code -32001

In all of the above cases, the server previously returned the default error
code -32000.
Co-authored-by: Nicholas Zhao <nicholas.zhao@gmail.com>
Co-authored-by: Felix Lange <fjl@twurst.com>