If sdk version specified in global.json is not found, latest sdk is used silently

[nguerrera]

Steps to reproduce

• Use a global.json specifying an SDK that is not installed.
• Check dotnet --version (or more generally, use any cli verb and see which sdk it uses)

D:\repro> dir /b "c:\Program Files\dotnet\sdk"
1.0.0
1.0.0-preview2-003121
1.0.0-preview2-003131
1.0.4
2.0.0-preview1-005957

D:\repro> type global.json
{
  "sdk": {
    "version": "1.2.0"
  }
}

D:\repro> dotnet --version
2.0.0-preview1-005957

Expected behavior

An error is reported indicating that the version specified in global.json is not present

Actual behavior

It just uses latest SDK instead (effectively ignoring global.json)

Scenarios

• Typos in global.json can get ignored.
• Teams can't impose a specific sdk version because on machines where that SDK is not installed, it will just use whatever is latest

Environment data

.NET Command Line Tools (2.0.0-preview1-005957)

Product Information:
 Version:            2.0.0-preview1-005957
 Commit SHA-1 hash:  056ac0e9cd

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.15063
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\2.0.0-preview1-005957\

Microsoft .NET Core Shared Framework Host

  Version  : 2.0.0-preview1-002111-00
  Build    : 1ff021936263d492539399688f46fd3827169983

@eerhardt @gkhanna79 @livarcocc @johnbeisner @dsplaisted

[gkhanna79]

@livarcocc Please triage for milestone and assign to appropriate individual.

[nguerrera]

This is higher priority than dotnet/core-setup#2336

[karelz]

This is blocking .NET Core 2.0 ZBB on 5/10.
If it is non-trivial I am fine moving it to Post-ZBB, given that it came quite late and close to the ZBB data.

@nguerrera @gkhanna79 any preferences?

[gkhanna79]

Livar should chime in.

[nguerrera]

It is not trivial enough to be filed on May 9 @ 5 PM and fixed by May 10. ;) Track it however appropriate, but it needs to be fixed. @johnbeisner is working on it.

[karelz]

OK, seems non-trivial. Given it came in late, I'm marking it as Post-ZBB.

[johnbeisner]

dotnet/core-setup@8a25b2a

[johnbeisner]

cherry-pick the changes to release/2.0.0