Convert step onError from string to "OnErrorType" type

Prior to this commit, the step onError field is defined as normal string and the 2 supported string values "continue" and "stopAndFail" are directly used across the codebase. This is error-prone and it introduces maintenance difficulty. This commit updates the onError field to be typed string "OnErrorType", with constants defined for the 2 supported values, and updates all the related references

cmd/entrypoint/main.go

@@ -30,6 +30,7 @@ import (
 	"github.com/containerd/containerd/platforms"
 	"github.com/tektoncd/pipeline/cmd/entrypoint/subcommands"
 	"github.com/tektoncd/pipeline/pkg/apis/pipeline"
+	"github.com/tektoncd/pipeline/pkg/apis/pipeline/v1beta1"
 	"github.com/tektoncd/pipeline/pkg/credentials"
 	"github.com/tektoncd/pipeline/pkg/credentials/dockercreds"
 	"github.com/tektoncd/pipeline/pkg/credentials/gitcreds"
@@ -146,7 +147,7 @@ func main() {
 		Results:             strings.Split(*results, ","),
 		Timeout:             timeout,
 		BreakpointOnFailure: *breakpointOnFailure,
-		OnError:             *onError,
+		OnError:             v1beta1.OnErrorType(*onError),
 		StepMetadataDir:     *stepMetadataDir,
 	}
 
@@ -175,12 +176,12 @@ func main() {
 			if status, ok := t.Sys().(syscall.WaitStatus); ok {
 				checkForBreakpointOnFailure(e, breakpointExitPostFile)
 				// ignore a step error i.e. do not exit if a container terminates with a non-zero exit code when onError is set to "continue"
-				if e.OnError != entrypoint.ContinueOnError {
+				if e.OnError != v1beta1.Continue {
 					os.Exit(status.ExitStatus())
 				}
 			}
 			// log and exit only if a step error must cause run failure
-			if e.OnError != entrypoint.ContinueOnError {
+			if e.OnError != v1beta1.Continue {
 				log.Fatalf("Error executing command (ExitError): %v", err)
 			}
 		default: