Some kinds of errors are returned as `%!s(PANIC=...nil pointer`

As described in https://github.com/filecoin-project/boost/issues/1193 when go-jsonrpc encounters some errors it turns them into `%!s(PANIC=...nil pointer`

lotus users have encountered this error message when making API calls that time out:

> ERROR: %!s(PANIC=Error method: runtime error: invalid memory address or nil pointer dereference)

The error message is printed out by [lotus cli helper](https://github.com/filecoin-project/lotus/blob/0c9f697bf611ad3a513d051a41e1288fa8859594/cli/helper.go#L52) when an error is returned from a CLI Action:
```
fmt.Fprintf(os.Stderr, "ERROR: %s\n\n", err)
```

The format of the error message `%!s(PANIC=...nil pointer` indicates that fmt.Fprintf encountered a nil pointer when trying to call the `Error()` method on the golang `error` object that was passed to it. This is most likely because the `error` object wraps a nil error. An example golang Test that reproduces this behaviour:
```
type myerr struct {
err error
}

func (e *myerr) Error() string {
// if e.err is nil, this will panic
return e.err.Error()
}

func TestPrintErr(t *testing.T) {
fmt.Printf("%s\n", &myerr{})
}

=== RUN TestPrintErr
%!s(PANIC=Error method: runtime error: invalid memory address or nil pointer dereference)
--- PASS: TestPrintErr (0.00s)
```

It looks like the underlying implementation of go-jsonrpc is returning an error object that wraps a nil error. My best guess is that this bug was introduced in https://github.com/filecoin-project/go-jsonrpc/pull/72