Tools: Improve message "Could not load assembly 'EFCore.Design'."

[bricelam]

There are a handful of ways you can encounter the following exception when using the tools, but the error doesn't provide any hints as to why you might be hitting it or what you can do to fix it.

System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.EntityFrameworkCore.Design, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.
File name: 'Microsoft.EntityFrameworkCore.Design, Culture=neutral, PublicKeyToken=null'
   at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks, IntPtr ptrLoadContextBinder)
   at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks, IntPtr ptrLoadContextBinder)
   at System.Reflection.Assembly.Load(AssemblyName assemblyRef)
   at Microsoft.EntityFrameworkCore.Tools.ReflectionOperationExecutor..ctor(String assembly, String startupAssembly, String projectDir, String contentRootPath, String dataDirectory, String rootNamespace, String environment)
   at Microsoft.EntityFrameworkCore.Tools.Commands.ProjectCommandBase.CreateExecutor()
   at Microsoft.EntityFrameworkCore.Tools.Commands.DbContextScaffoldCommand.Execute()
   at Microsoft.DotNet.Cli.CommandLine.CommandLineApplication.Execute(String[] args)
   at Microsoft.EntityFrameworkCore.Tools.Program.Main(String[] args)

We should catch it and re-throw a more helpful message.

A few reasons I've seen people hitting it:

• They don't have Microsoft.EntityFrameworkCore.Design (or Tools) installed in their startup project.
• They're inadvertently using the wrong startup project; they don't know the Debug startup project affects the PMC commands.
• They're using packages.config and even though Microsoft.EntityFrameworkCore.Design is referenced transitively, MSBuild won't copy it into the output directory unless you reference it directly.