Implement err.WithCause(), a shorter version of proto.ErrorWithCause() (#42)

* Implement err.WithCause(), a shorter version of proto.ErrorWithCause()

* Deprecate proto.ErrorWithCause()

Author: VojtechVitek

_examples/golang-basics/example.gen.go

@@ -45,8 +45,8 @@ func (e WebRPCError) Unwrap() error {
 	return e.cause
 }
 
-func ErrorWithCause(rpcErr WebRPCError, cause error) WebRPCError {
-	err := rpcErr
+func (e WebRPCError) WithCause(cause error) error {
+	err := e
 	err.cause = cause
 	err.Cause = cause.Error()
 	{{- if $opts.errorStackTrace }}
@@ -55,6 +55,11 @@ func ErrorWithCause(rpcErr WebRPCError, cause error) WebRPCError {
 	return err
 }
 
+// Deprecated: Use .WithCause() method on WebRPCError.
+func ErrorWithCause(rpcErr WebRPCError, cause error) WebRPCError {
+	return rpcErr.WithCause(cause)
+}
+
 {{- if $opts.errorStackTrace -}}
 func (e WebRPCError) StackFrames() []uintptr {
 	return e.frame.frames[:]