refactor(errs): migrate from gperr.Error to standard Go error interface

This is a large-scale refactoring across the codebase that replaces the custom
`gperr.Error` type with Go's standard `error` interface. The changes include:

- Replacing `gperr.Error` return types with `error` in function signatures
- Using `errors.New()` and `fmt.Errorf()` instead of `gperr.New()` and `gperr.Errorf()`
- Using `%w` format verb for error wrapping instead of `.With()` method
- Replacing `gperr.Subject()` calls with `gperr.PrependSubject()`
- Converting error logging from `gperr.Log*()` functions to zerolog's `.Err().Msg()` pattern
- Update NewLogger to handle multiline error message
- Updating `goutils` submodule to latest commit

This refactoring aligns with Go idioms and removes the dependency on
custom error handling abstractions in favor of standard library patterns.

internal/net/gphttp/loadbalancer/loadbalancer.go

@@ -56,7 +56,7 @@ func New(cfg *types.LoadBalancerConfig) *LoadBalancer {
 }
 
 // Start implements task.TaskStarter.
-func (lb *LoadBalancer) Start(parent task.Parent) gperr.Error {
+func (lb *LoadBalancer) Start(parent task.Parent) error {
 	lb.startTime = time.Now()
 	lb.task = parent.Subtask("loadbalancer."+lb.Link, true)
 	lb.task.OnCancel("cleanup", func() {
@@ -234,7 +234,7 @@ func (lb *LoadBalancer) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
 			})
 		}
 		if err := errs.Wait().Error(); err != nil {
-			gperr.LogWarn("failed to wake some servers", err, &lb.l)
+			lb.l.Warn().Err(err).Msg("failed to wake some servers")
 		}
 	}