Python CodeGen should provide clear message for self-service trouble shooting

### Clear and concise description of the problem

We need to empower our customers both internal and external to easily trouble shooting and fix spec issues that would block the CodeGen.
If certain TypeSpec features or patterns are not supported by CodeGen, we should clearly point it out and suggest them to update the spec with the features we supported for CodeGen.

Today, Python CodeGen just returned a big call-stack like below which could not help for self-service resolving the problem.

I suggest we improve the following areas:

1. Leverage TypeSpec diagnostics APIs to report the errors and provide more context so that users could easily identify the TypeSpec source that cause the error.
2. Provide a clear message for the unsupported scenarios and ideally provide some suggestions on how to fix the spec.

The error reported today (copied from https://github.com/Azure/autorest.python/issues/2975#issuecomment-2574201917):
```
ExternalError: Emitter "@azure-tools/typespec-python" crashed! This is a bug.
Please file an issue at https://github.com/Azure/autorest.python/issues

Error: Command failed: /home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator/TempTypeSpecFiles/apiview-treestyle-parser-schema/node_modules/@typespec/http-client-python/venv/bin/python /home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator/TempTypeSpecFiles/apiview-treestyle-parser-schema/node_modules/@typespec/http-client-python/eng/scripts/setup/run_tsp.py --output-folder=/home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator --cadl-file=/tmp/cadl-codegen/python-yaml-path3e7a3279-3ef7-4a45-9b82-59d8fad79e04.yaml --package-version=0.3.13 --generate-packaging-files=false --flavor=azure --package-dir=apiview-stub-generator --package-name=treestyle-parser --generate-test=false --generate-sample=false --models-mode=dpg --emit-cross-language-definition-file=true --from-typespec=true
Traceback (most recent call last):
File "/home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator/TempTypeSpecFiles/apiview-treestyle-parser-schema/node_modules/@typespec/http-client-python/eng/scripts/setup/run_tsp.py", line 41, in
codegen.CodeGenerator(output_folder=args.output_folder, cadl_file=args.cadl_file, **unknown_args).process()
File "/home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator/TempTypeSpecFiles/apiview-treestyle-parser-schema/node_modules/@typespec/http-client-python/generator/pygen/codegen/__init__.py", line 336, in process
self.sort_exceptions(yaml_data)
File "/home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator/TempTypeSpecFiles/apiview-treestyle-parser-schema/node_modules/@typespec/http-client-python/generator/pygen/codegen/__init__.py", line 247, in sort_exceptions
for group in client["operationGroups"]:
KeyError: 'operationGroups'

at genericNodeError (node:internal/errors:983:15)
at wrappedFn (node:internal/errors:537:14)
at checkExecSyncError (node:child_process:882:11)
at execSync (node:child_process:954:15)
at $onEmit (file:///home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator/TempTypeSpecFiles/apiview-treestyle-parser-schema/node_modules/@typespec/http-client-python/dist/emitter/emitter.js:110:9)
at async Object.$onEmit [as emitFunction] (file:///home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator/TempTypeSpecFiles/apiview-treestyle-parser-schema/node_modules/@azure-tools/typespec-python/dist/src/emitter.js:3:5)
at async runEmitter (file:///home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator/TempTypeSpecFiles/apiview-treestyle-parser-schema/node_modules/@typespec/compiler/dist/src/core/program.js:366:13)
at async compile (file:///home/swathip/repos/azure-sdk-tools/packages/python-packages/apiview-stub-generator/TempTypeSpecFiles/apiview-treestyle-parser-schema/node_modules/@typespec/compiler/dist/src/core/program.js:123:9)
at async compileTsp (file:///home/swathip/.nvm/versions/node/v23.5.0/lib/node_modules/@azure-tools/typespec-client-generator-cli/dist/typespec.js:97:21)
at async generateCommand (file:///home/swathip/.nvm/versions/node/v23.5.0/lib/node_modules/@azure-tools/typespec-client-generator-cli/dist/commands.js:248:35)

```

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.