RunTemplateAsync returns TemplateProcessingException

## File a bug

`RazorTemplating.RunTemplateAsync` *returns* a `TemplateProcessingException` instead of throwing it. My explaining why this is a bad thing would be an insult to your intelligence.

Fixing this bug involves getting rid of class `TemplateResult` so it is not a one-liner.

### Include your code

```C#
if (templateResult.Messages.Any())
{
/* this is good:
throw
new TemplateProcessingException
(templateResult.Messages, generatorResults.GeneratedCode);
This is bad: */
return new TemplateResult()
{
GeneratedText = string.Empty,
ProcessingException = new TemplateProcessingException(templateResult.Messages, generatorResults.GeneratedCode)
};
}

```

### Include stack traces

Include the full exception message and stack trace for any exception you encounter.

Use triple-tick fences for stack traces. For example:

```
at Microsoft.VisualStudio.Web.CodeGeneration.Templating.RazorTemplating.RunTemplateAsync(String content, Object templateModel) in /home/dell/prog/Scaffolding/src/Scaffolding/VS.Web.CG.Templating/RazorTemplating.cs:line 74
at Microsoft.VisualStudio.Web.CodeGeneration.CodeGeneratorActionsService.AddFileFromTemplateAsync(String outputPath, String templateName, IEnumerable`1 templateFolders, Object templateModel) in /home/dell/prog/Scaffolding/src/Scaffolding/VS.Web.CG.Core/CodeGeneratorActionsService.cs:line 85
at Microsoft.VisualStudio.Web.CodeGenerators.Mvc.Razor.RazorPageScaffolderBase.GenerateView(RazorPageGeneratorModel razorGeneratorModel, ModelTypeAndContextModel modelTypeAndContextModel, String outputPath) in /home/dell/prog/Scaffolding/src/Scaffolding/VS.Web.CG.Mvc/RazorPage/RazorPageScaffolderBase.cs:line 217
at Microsoft.VisualStudio.Web.CodeGenerators.Mvc.Razor.EmptyRazorPageScaffolder.GenerateCode(RazorPageGeneratorModel razorGeneratorModel) in /home/dell/prog/Scaffolding/src/Scaffolding/VS.Web.CG.Mvc/RazorPage/EmptyRazorPageScaffolder.cs:line 49
at Microsoft.VisualStudio.Web.CodeGenerators.Mvc.Razor.RazorPageGenerator.GenerateCode(RazorPageGeneratorModel razorPageGeneratorModel) in /home/dell/prog/Scaffolding/src/Scaffolding/VS.Web.CG.Mvc/RazorPage/RazorPageGenerator.cs:line 65
```

### Include provider and version information

Microsoft.VisualStudio.Web.CodeGeneration.Design/Microsoft.DotNet.MSIdentity version:

1.0.0

Target framework: (e.g. .NET 5.0)

net6.0

Operating system:

Linux x64 5.15.7-1-default

IDE: (e.g. Visual Studio 2019 16.3)

1.63.2