Improve scripted System issue report

[AndreaCatania]

When a scripted System doesn't include one of the methods _prepare or _execute, the System fails silently, and isn't included by the available systems, and is impossible to add it to the pipeline.

This make it hard to find issues: as proposal would be great if the System is available to be added on the pipeline as soon as a script is created (even if invalid), and when used an error is displayed on the PipelineBuilder error output screen, that explains how to fix the issue.

As an example is possible to check this: https://twitter.com/_AndreaCatania/status/1408006585155264512