fix: improve background-worker boot-failure logs

Background workers inherited the HTTP-worker wording which mentioned
frankenphp_handle_request — that function doesn't apply to bg workers.
The ready signal for a bg worker is frankenphp_set_vars.

- startupFailChan error: "background worker %s has not reached frankenphp_set_vars()"
- Warn log (watcher): "(watcher enabled) background worker has not reached frankenphp_set_vars()"
- Warn log (normal): "background worker boot failed, restarting"

The log now also carries exit_status and, when captured, the last PHP
error message from bootFailureInfo. Tailing the FrankenPHP log at Warn
level is enough to see what went wrong without reading the PHP error
log separately.

Author: nicolas-grekas

threadbackgroundworker.go

@@ -206,18 +206,24 @@ func (handler *backgroundWorkerThread) afterScriptExecution(exitStatus int) {
 	}
 
 	if worker.maxConsecutiveFailures >= 0 && startupFailChan != nil && !watcherIsEnabled && handler.failureCount >= worker.maxConsecutiveFailures {
-		startupFailChan <- fmt.Errorf("too many consecutive failures: worker %s has not reached frankenphp_handle_request()", worker.fileName)
+		startupFailChan <- fmt.Errorf("too many consecutive failures: background worker %s has not reached frankenphp_set_vars()", worker.fileName)
 		handler.thread.state.Set(state.ShuttingDown)
 		return
 	}
 
+	attrs := []slog.Attr{slog.String("worker", worker.name), slog.Int("thread", handler.thread.threadIndex), slog.Int("failures", handler.failureCount), slog.Int("exit_status", exitStatus)}
+	if worker.backgroundWorker != nil {
+		if info := worker.backgroundWorker.bootFailure.Load(); info != nil && info.phpError != "" {
+			attrs = append(attrs, slog.String("error", info.phpError))
+		}
+	}
 	if watcherIsEnabled {
 		if globalLogger.Enabled(globalCtx, slog.LevelWarn) {
-			globalLogger.LogAttrs(globalCtx, slog.LevelWarn, "(watcher enabled) worker script has not reached frankenphp_handle_request()", slog.String("worker", worker.name), slog.Int("thread", handler.thread.threadIndex))
+			globalLogger.LogAttrs(globalCtx, slog.LevelWarn, "(watcher enabled) background worker has not reached frankenphp_set_vars()", attrs...)
 		}
 	} else {
 		if globalLogger.Enabled(globalCtx, slog.LevelWarn) {
-			globalLogger.LogAttrs(globalCtx, slog.LevelWarn, "worker script has failed on restart", slog.String("worker", worker.name), slog.Int("thread", handler.thread.threadIndex), slog.Int("failures", handler.failureCount))
+			globalLogger.LogAttrs(globalCtx, slog.LevelWarn, "background worker boot failed, restarting", attrs...)
 		}
 	}