Synth: Drive application flow based on missing resources

### Describe the feature

Be able to manage (try/catch ?) missing resources from lookups, to be able to skip sections on a first pass if they don't exist yet.

### Use Case

At the moment the way that I handle missing resources is to have configuration options, and fill in the values after the resources have been created. The creation of dependent resources is then based on the existence of a value in the configuration.

This is done to avoid Import/Export parameters that create a hard dependency between stacks. When there is a complex set of stacks and a structural change is needed, it is often the case that the dependent stack need to be manually deleted if an export parameter would be deleted in the new structure.

### Proposed Solution

Optionally throw an exception on lookup failures. Alternatively a null object could be returned from the lookup.

I would see this a configurable option that could be turned on if desired.

### Other Information

I am open to helping to make this happen. I would just need a little support in getting started. I have previously stood up the repository in a dev container, but the processes and structure is a lot to digest.

### Acknowledgements

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

### CDK version used

2.130.0 (build bd6e5ee)

### Environment details (OS name and version, etc.)

Windows 11 22H2 22621.3007

The issue names no files or tests; start by locating the lookup APIs and existing missing-resource handling in the repository. Read how dependent stacks and Import/Export parameters are represented, then establish whether configurable exceptions or a null object is intended. Done should include defined behavior for missing lookups and coverage of the first-pass stack flow.

Written by the indexing model from the issue text.