When POST requests in the streamableClientConn return a transient error,
return this error to the caller rather than permanently breaking the
connection.
This is achieved by using the special sentinel ErrRejected error to the
jsonrpc2 layer. In doing so, the change revealed a pre-existing bug:
ErrRejected had the same code as ErrConnectionClosing, and
jsonrpc2.WireError implements errors.Is, so the two sentinel values
could be conflated. This is fixed by using a new internal code.
There's more to do for #683: we should also retry transient errors in
handleSSE.
For #683
875d1d30c2