Trying to run preview 3 commands on project.json projects could give clearer error

Steps to reproduce

1. Create a project.json-based project.
2. Run a preview 3 command like dotnet restore, dotnet build or dotnet run on it.

Expected behavior

Clear error message explaining that the project is not compatible and suggesting dotnet migrate.

Actual behavior

>dotnet restore
MSBUILD : error MSB1003: Specify a project or solution file. The current working directory does not contain a project or solution file.

>dotnet build
Microsoft (R) Build Engine version 15.1.0.0
Copyright (C) Microsoft Corporation. All rights reserved.

MSBUILD : error MSB1003: Specify a project or solution file. The current working directory does not contain a project or solution file.

>dotnet run
Couldn't find a project to run. Ensure a project exists in E:\Users\Svick\git\core\samples\helloworld.
Or pass the path to the project using --project

This is confusing to users that don’t know the difference between the various Preview versions or about the switch from project.json to msbuild, for example: https://github.com/dotnet/core/issues/363.

Environment data

dotnet --info output:

.NET Command Line Tools (1.0.0-preview3-004056)

Product Information:
 Version:            1.0.0-preview3-004056
 Commit SHA-1 hash:  ccc4968bc3

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.14393
 OS Platform: Windows
 RID:         win10-x64

[vcsjones]

+1.

I would propose:

1. If there is no csproj / sln, and there is a project.json or global.json, then print out a message about “dotnet migrate”
2. If there is no project.json, then the existing message is probably fine.
3. If there is a csproj and a project.json, the tooling should be happy to use whatever it wants to so users can have SxS tooling for a project, if they want.

[livarcocc]

This seems like a duplicate of https://github.com/dotnet/cli/issues/4628.