Give clear error about a malformed config or template.yaml when loading the Mock Lambda Test Tool

### Describe the feature

If the template.yaml is invalid, let the test tool show me an error rather than just not load any configs or functions into the dropdowns.

### Use Case

If the Mock Lambda Test Tool fails to load, it doesn't really give a clear indicator as to why. In my case, I had an extra space in my template.yaml, but I thought it was a complete different problem for almost a whole day. Probably a rookie mistake, but would appreciate a clear helpful error to tell me next time.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS .NET SDK and/or Package version used

.NET 6 Lambda Test Tool

### Targeted .NET Platform

.NET 6

### Operating System and version

Windows 11

Riproduci il problema in Mock Lambda Test Tool con il template.yaml malformato descritto nell’issue, quindi traccia il percorso di caricamento che popola i menu a discesa config e function. Non vengono indicati file sorgente o test; il lavoro è completato quando un template non valido produce un errore chiaro e utile invece di lasciare i menu a discesa vuoti senza alcuna indicazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.