chore(pipelines): reference discoverReferencedOutputs in error message (#19176)

The need to call this function is referenced in the README, but if you
already have written custom steps you're not going to read the README
again.

Include the same hints in the error message.

Fixes #19173.


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rix0rrr

packages/@aws-cdk/pipelines/README.md

@@ -801,7 +801,7 @@ class MyJenkinsStep extends pipelines.Step implements pipelines.ICodePipelineAct
   ) {
     super('MyJenkinsStep');
 
-    // This is necessary if your step accepts things like environment variables
+    // This is necessary if your step accepts parametres, like environment variables,
     // that may contain outputs from other steps. It doesn't matter what the
     // structure is, as long as it contains the values that may contain outputs.
     this.discoverReferencedOutputs({

packages/@aws-cdk/pipelines/lib/blueprint/step.ts

@@ -105,10 +105,10 @@ export abstract class Step implements IFileSetProducer {
   /**
    * Crawl the given structure for references to StepOutputs and add dependencies on all steps found
    *
-   * Should be called by subclasses based on what the user passes in as
-   * construction properties. The format of the structure passed in here does
-   * not have to correspond exactly to what gets rendered into the engine, it
-   * just needs to contain the same amount of data.
+   * Should be called in the constructor of subclasses based on what the user
+   * passes in as construction properties. The format of the structure passed in
+   * here does not have to correspond exactly to what gets rendered into the
+   * engine, it just needs to contain the same data.
    */
   protected discoverReferencedOutputs(structure: any) {
     for (const output of StepOutput.findAll(structure)) {

packages/@aws-cdk/pipelines/lib/helpers-internal/step-output.ts

@@ -104,7 +104,7 @@ export class StepOutput implements IResolvable {
 
   public resolve(_context: IResolveContext) {
     if (this.resolution === undefined) {
-      throw new Error(`Output for step ${this.step} not configured. Either the step is not in the pipeline, or this engine does not support Outputs for this step.`);
+      throw new Error(`Output for step ${this.step} not configured. Either the step is not in the pipeline, the step implementation did not call 'this.discoverReferencedOutputs()', or this engine does not support Outputs for this step.`);
     }
     return this.resolution;
   }