Forward error types as their string representation (#131)

By default go serializes `error` types as an empty struct `{}`, this
patch would adjust that so that errors are serialized as their string
representation by calling `Error()` on any non-nil types that implement
the error interface.

This should be an improvement over the current state as it now allows
identifying the cause of an error from the RPC client. An alternative
could be to serialize the error as a struct of `{"type": <>, "message":
<>}` using reflection to get the type name (and package path) of the
error though I'm not sure if it's necessary and I'd have to check how
easy that would be to get working.

examples/remote/util.go

@@ -25,11 +25,23 @@ func isExportedOrBuiltinType(t reflect.Type) bool {
 	return token.IsExported(t.Name()) || t.PkgPath() == ""
 }
 
+// json.Marshall won't marshall error types, marshall as string
+func errorAsJson(v reflect.Value) interface{} {
+	if v.IsNil() {
+		// passthrough nil as nil, otherwise nil.(error) will panic
+		return nil
+	} else {
+		return v.Interface().(error).Error()
+	}
+}
+
 func transformReturnValues(values []reflect.Value) []interface{} {
 	result := make([]interface{}, len(values))
 
 	for i, e := range values {
-		switch e.Type() {
+		valueType := e.Type()
+
+		switch valueType {
 		case reflect.TypeFor[spineapi.DeviceRemoteInterface]():
 			result[i] = e.Interface().(spineapi.DeviceRemoteInterface).Address()
 		case reflect.TypeFor[[]spineapi.DeviceRemoteInterface]():
@@ -51,12 +63,18 @@ func transformReturnValues(values []reflect.Value) []interface{} {
 			}
 			result[i] = transformedValues
 		default:
-			result[i] = e.Interface()
+			switch {
+			case valueType.Implements(reflect.TypeFor[error]()):
+				result[i] = errorAsJson(e)
+			default:
+				result[i] = e.Interface()
+			}
 		}
 	}
 
 	return result
 }
+
 func WriteKey(cert tls.Certificate, path string) error {
 	file, err := os.Create(path)
 	if err != nil {