Add solution source generator path from attribute support and exception reporting

[jwfx]

Adds the missing support for supplying the solution name via attribute parameter instead of parameter.json while having source generation enabled, e.g.
[Solution("my.sln", GenerateProjects = true)] readonly Solution Solution;

Prevent source generator exceptions from being swallowed and replaced by unhelpful generic compiler errors by using the proper reporting facility context.ReportDiagnostic().

Fixes #1205

I confirm that the pull-request:

• Follows the contribution guidelines
• Is based on my own work
• Is in compliance with my employer

[matkoch]

Can you please split this in two PRs, or at least two commits? The current one does two things at the same time.

For the exception handling I suggest introducing another Execute overload that takes the compilation. This way, the error handling is a single response of the outer method, and there's not a huge indented block as it is now.

[jwfx]

Can you please split this in two PRs, or at least two commits? The current one does two things at the same time.

Done, split it in two commits.

For the exception handling I suggest introducing another Execute overload that takes the compilation. This way, the error handling is a single response of the outer method, and there's not a huge indented block as it is now.

I turned the source generation into a method. It felt a bit awkward to make an overload that would take the context and the compilation from the same context. The context would still have been required because of the AddSource() call.

[matkoch]

This is perfect! Just as I would've done it.