Warnings and errors should include more context when possible

[sbomer]

As we surface linker warnings/errors to users (see https://github.com/mono/linker/pull/1105 for example), we should revisit the messages to include more context when reasonable, and ensure that they convey the right amount of information.

For example:

• Errors about unresolved things in XML descriptors should at least say which descriptor had the error, and maybe even report the element (or XML line number if we can get it) that caused it. Currently we silently move on when a type doesn't resolve for example.
• The unrecognized reflection messages are pretty verbose - we may want to include less information in the default message as long as there is a way to get more context.

[marek-safar]

It'd be great to add links to a few examples you found

[sbomer]

If I get more time, I'd like to do a more exhaustive pass, but here are two examples for now:

• IL2001 Type 'type' has no fields to preserve should mention where the preserve came from
• IL2007 Could not match assembly 'assembly' for substitution should mention where the substitution came from

I added links to the above, too.

[sbomer]

This issue also contains a related ask for better diagnostics: https://github.com/dotnet/aspnetcore/issues/20145

The build log doesn't really why it was trying to resolve that assembly:

Fatal error in Mono IL Linker
C:\Users\…\.nuget\packages\microsoft.aspnetcore.components.webassembly.build\3.2.0-preview2.20160.5\targets\Blazor.MonoRuntime.targets(245,5): error : Unhandled exception. Mono.Linker.Steps.XmlResolutionException: Failed to process XML description: <unspecified> [C:\agent\_work\19\s\MyBlazorProject.Client\MyBlazorProject.Client.csproj]
---> Mono.Cecil.AssemblyResolutionException: Failed to resolve assembly: 'Microsoft.AspNetCore.Blazor, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'
---> Mono.Cecil.AssemblyResolutionException: Failed to resolve assembly: 'Microsoft.AspNetCore.Blazor, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'
  at Mono.Cecil.BaseAssemblyResolver.Resolve(AssemblyNameReference name, ReaderParameters parameters)
  at Mono.Linker.AssemblyResolver.Resolve(AssemblyNameReference name, ReaderParameters parameters)
  at Mono.Linker.LinkContext.Resolve(IMetadataScope scope)
  at Mono.Linker.LinkContext.Resolve(IMetadataScope scope)
  at Mono.Linker.Steps.ResolveFromXmlStep.ProcessAssemblies(LinkContext context, XPathNodeIterator iterator)
  at Mono.Linker.Steps.ResolveFromXmlStep.Process()
  --- End of inner exception stack trace ---
  at Mono.Linker.Steps.ResolveFromXmlStep.Process()
  at Mono.Linker.Steps.BaseStep.Process(LinkContext context)
  at Mono.Linker.Pipeline.ProcessStep(LinkContext context, IStep step)
  at Mono.Linker.Pipeline.Process(LinkContext context)
  at Mono.Linker.Driver.Run(ILogger customLogger)
  at Mono.Linker.Driver.Execute(String[] args, ILogger customLogger)
  at Mono.Linker.Driver.Main(String[] args)