### Motivation and Context

<!-- Thank you for your contribution to the semantic-kernel repo!
Please help reviewers and future users, providing the following
information:
  1. Why is this change required?
  2. What problem does it solve?
  3. What scenario does it contribute to?
  4. If it fixes an open issue, please link to the issue here.
-->

Resolves microsoft#4440

This PR adds a more detailed `HandlebarsPlanCreationException` type,
which allows users to inspect the prompt and model results, if
available.

Als includes a minor update to sample to add clarifying comments.

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

Users can catch `HandlebarsPlanCreationException` to inspect the prompt,
proposed plan, and exception details on error. All exceptions will be
bubbled up to the caller.

```
var planner = new HandlebarsPlanner();

try
{
     var plan = await planner.CreatePlanAsync(kernel, intent, cancellationToken);
}
catch (HandlebarsPlanCreationException ex)
{
     Console.WriteLine(ex.Message);
     Console.WriteLine(ex.InnerException?.Message);
     Console.WriteLine($"CreatePlan Prompt: {ex.CreatePlanPrompt}");
     Console.WriteLine($"Proposed plan (model output): {ex.ModelResults.Content}");
     throw ex;
}
```

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

### Motivation and Context

<!-- Thank you for your contribution to the semantic-kernel repo!
Please help reviewers and future users, providing the following
information:
  1. Why is this change required?
  2. What problem does it solve?
  3. What scenario does it contribute to?
  4. If it fixes an open issue, please link to the issue here.
-->

Resolves microsoft#4440

This PR adds a more detailed `HandlebarsPlanCreationException` type,
which allows users to inspect the prompt and model results, if
available.

Als includes a minor update to sample to add clarifying comments.

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

Users can catch `HandlebarsPlanCreationException` to inspect the prompt,
proposed plan, and exception details on error. All exceptions will be
bubbled up to the caller.

```
var planner = new HandlebarsPlanner();

try
{
     var plan = await planner.CreatePlanAsync(kernel, intent, cancellationToken);
}
catch (HandlebarsPlanCreationException ex)
{
     Console.WriteLine(ex.Message);
     Console.WriteLine(ex.InnerException?.Message);
     Console.WriteLine($"CreatePlan Prompt: {ex.CreatePlanPrompt}");
     Console.WriteLine($"Proposed plan (model output): {ex.ModelResults.Content}");
     throw ex;
}
```

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄