Emit an actionable error when Microsoft.WebApplication.targets / WebApplicationsTargetPath cannot be resolved

[ehasis]

Split out from #103 (item 2).

Summary

When WebApplicationsTargetPath resolves to a path that does not exist (the common case under dotnet, where $(MSBuildExtensionsPath32) points at the .NET SDK instead of a Visual Studio install), the build fails with a cryptic, low-level error:

error MSB4019: The imported project "...\dotnet\sdk\x.y.z\Microsoft\VisualStudio\v18.0\WebApplications\Microsoft.WebApplication.targets" was not found.

Proposed change

Make the Microsoft.WebApplication.targets import conditional on Exists('$(WebApplicationsTargetPath)') and add an early target that emits a clear, actionable error (e.g. SYSWEB001) explaining how to fix it: install the Visual Studio / Build Tools "web development build tools" workload, build from a Visual Studio Developer command prompt, or set VSToolsPath / WebApplicationsTargetPath explicitly (e.g. computed via VSWhere in a CI pre-build step).

Note

This is intended as interim, in-SDK guidance. If Microsoft.WebApplication.targets becomes available as a separately packaged/released component from Microsoft (cc @twsouthwick), that would likely be the better long-term source and this could defer to it. See #104.

[CZEMacLeod]

I think it might be good to add a property such as MissingWebApplicationsTargetPathAction which would default to Error.
Possible options might be:

Value	Description
Ignore	Do nothing
Skip	Write an informational message only
Warn	Write a warning message
Error	Output an error and fail

I think a list of all the possible error/warning messages with ID's should be added to the Wiki and/or docs and all the messages updated to include error numbers. I've been meaning to do this for a while now.