Enhanced error message for non-existent Scheduled Pipelines

There is unclear error message when a scheduled custom pipeline is not found in the bitbucket-pipelines.yml file. Currently, the error message suggests that the bitbucket-pipelines.yml file itself is missing, rather than indicating the absence of the specific requested pipeline within the file.

Steps to Reproduce:

1. Create a bitbucket-pipelines.yml file with specific custom pipeline definitions.
2. Create a "Scheduled Pipelines" that is not defined in the bitbucket-pipelines.yml file, either due to a case mismatch or an entirely different name via an API call.

Actual Result:

"Scheduled execution failed because no bitbucket-pipelines.yml was found." This message implies that the bitbucket-pipelines.yml file is missing or not found, which is not the case when the file exists but lacks the specified pipeline.

Expected Result:

The error message should reflect the issue: "Pipeline '[pipeline-name]' not found in bitbucket-pipelines.yml." This message would immediately inform the user that the specific requested pipeline is not defined in the file, guiding them to check the pipeline names or their cases in the yml file.