Error shorthand WithCausef (e.WithCause + fmt.Errof) (#65)

* Update errors.go.tmpl

* Update errors.go.tmpl

* gen files

* generate with 15.2

* use webrpc built from scratch as CI

* use interface

* errorStackTrace support

Author: klaidliadon

_examples/golang-basics/example.gen.go

@@ -58,6 +58,17 @@ func (e WebRPCError) WithCause(cause error) WebRPCError {
 	return err
 }
 
+func (e WebRPCError) WithCausef(fmt string, args ...interface{}) WebRPCError {
+	cause := fmt.Errorf(fmt, args...)
+	err := e
+	err.cause = cause
+	err.Cause = cause.Error()
+	{{- if $opts.errorStackTrace }}
+	runtime.Callers(1, err.frame.frames[:])
+	{{- end}}
+	return err
+}
+
 // Deprecated: Use .WithCause() method on WebRPCError.
 func ErrorWithCause(rpcErr WebRPCError, cause error) WebRPCError {
 	return rpcErr.WithCause(cause)