better way to show reinstall after framework changes when restore/build with auto-detect

Description

Can we show some information to inform users re-install paket dependency, when target framework changes on project file.

When install paket.lock with auto-detect, it will generate the restrictions depend on project files's TargetFramework(s) . After install, upgrade TargetFramework in project file, then rebuild project, paket will generate an empty projectxxx.frameworkxxx.paket.resolved and empty projectxxx.references file. So dotnet build will falied to load the right dependencies, and show build errors.

Repro steps

Please provide the steps required to reproduce the problem

• framework: auto-detect in paket.dependencies
• paket install to have a 'paket.lock' file
• change TargetFramework netcoreapp2.1 -> netcoreapp2.2
• dotnet build ( Errors Here )

Expected behavior

can we show some info to users, they need re-install paket.lock before execute msbuild.

Actual behavior

Show build errors, may lead some confusions.

Known workarounds

paket install after change project file target frameworks

Start by reproducing the flow with framework: auto-detect in paket.dependencies, a paket.lock file, and a TargetFramework change from netcoreapp2.1 to netcoreapp2.2, then run dotnet build. Trace how the project file, paket.resolved, and references files are handled; done means users receive clear guidance to run paket install before building when frameworks change.