Warnings and errors should include more context when possible

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](https://github.com/mono/linker/blob/dff175e3758610101deaa2cc6f106078892971b0/src/linker/Linker.Steps/ResolveFromXmlStep.cs#L185) for example.
- The unrecognized reflection [messages](https://github.com/mono/linker/blob/dff175e3758610101deaa2cc6f106078892971b0/test/Mono.Linker.Tests.Cases/DataFlow/LocalDataFlow.cs#L64) 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.