Use a better error message when TargetFramework property has a semicolon in it

[dsplaisted]

Fixes #385

For checking for a TargetFramework value with a semicolon in it, this reuses the existing check that TargetFrameworkIdentifier or TargetFrameworkVersion couldn't be inferred, but uses a different error message:

The TargetFramework value 'netcoreapp2.0;net46' is not valid. To multi-target, use the 'TargetFrameworks' property instead.

Fixes #1015

Updates the error message in the case where there isn't a semicolon in TargetFramework to:

The TargetFramework value 'foo45' was not recognized. It may be misspelled. If not, then the TargetFrameworkIdentifier and/or TargetFrameworkVersion properties must be specified explicitly.

This PR also updates the _CheckForUnsupportedTargetFramework target to run before Restore, since a valid target framework is necessary in order to restore.

[dsplaisted]

@dotnet-bot
test Windows_NT Release
test Windows_NT_FullFramework Debug

[nguerrera]

IIRC, this won't run before restore unless it's the root project. We use a private target (_GenerateRestoreSpec or something, search for it) with @emgarten's approval elsewhere.

[emgarten]

GenerateRestoreProjectSpec is the per project outer build target that restore executes. As mentioned restore is only called once and that might be in the root project or in the solution metaproj depending on how the restore was done.

https://github.com/NuGet/NuGet.Client/blob/dev/src/NuGet.Core/NuGet.Build.Tasks/NuGet.targets#L423

[nguerrera]

This needs to go in inner build as it's validating targetframework singular. Do we need to intercept restore? Doesn't restore already have a good enough error for bad targetframework?

[dsplaisted]

The message from restore is:

error : Invalid restore input. Invalid target framework 'unsupported'. Input files: C:\git\dotnet-sdk\bin\repro\WrongFramework\WrongFramework.csproj.

Note that it says that the target framework is "unsupported" for any invalid value of TargetFramework.

Also, in the case that you have multiple valid target frameworks separated by semicolons in the TargetFramework property, the restore operation appears to just work, restoring for both.

[nguerrera]

I don't see tests for both cases.

[dsplaisted]

I've added both tests

[dsplaisted]

@MattGertz for approval

Customer scenario

Setting the TargetFramework in the project file. This PR improves the error messages that are generated.

Bugs this fixes:

#385 and #1015

Workarounds, if any

n/a - This change makes error messages more helpful

Risk

Low - This changes the text of an error, and adds a different error message in a special case

Performance impact

Low - a few extra conditional evaluations during build

Is this a regression from a previous update?

No

Root cause analysis:

n/a

How was the bug found?

dogfooding