Revise single workflow exit path

- Added log message before canceling in error case.
- Return after calling ctxCancel.
- This makes the control flow for the single-workflow case match what
  it was in the original implementation of woodpecker-ci#4019.  As of commit
  5c9fc61, the 'err' value is never
  propagated to the caller, so this implementation returns nil.
- Fixed grammar in startup log message.

Author: cliffmccarthy

cmd/agent/core/agent.go

@@ -304,7 +304,9 @@ func run(ctx context.Context, c *cli.Command, backends []types.Backend) error {
 				log.Debug().Msg("polling new workflow")
 				if err := runner.Run(agentCtx, shutdownCtx); err != nil {
 					if singleWorkflow {
+						log.Error().Err(err).Msg("runner done with error")
 						ctxCancel(nil)
+						return nil
 					}
 					log.Error().Err(err).Msg("runner error, retrying...")
 					// Check if context is canceled
@@ -331,7 +333,7 @@ func run(ctx context.Context, c *cli.Command, backends []types.Backend) error {
 
 	if singleWorkflow {
 		log.Info().Msgf(
-			"starting Woodpecker agent with version '%s' and backend '%s' using platform '%s' running up in single workflow mode",
+			"starting Woodpecker agent with version '%s' and backend '%s' using platform '%s' running in single workflow mode",
 			version.String(), backendEngine.Name(), engInfo.Platform)
 	} else {
 		log.Info().Msgf(