MSBuild failures/warnings returns zero exit code

I do not know of any other way to trigger this. Hence, I will demonstrate with [Paket](https://github.com/fsprojects/Paket) and missing references.

**Reproduction**
1. Clone this repo: https://github.com/Arthri/dotnet-format-missingref. This repo cuts the steps required, but to create a similar repo, refer to Reproduction Full.
2. Run `dotnet tool restore`
3. Do NOT run `dotnet paket restore`. Pretend this is a mistake
4. Run `dotnet format MissingRef.sln`. The command fails(output is below) to format anything, but returns a zero exit code indicating it succeeded. Therefore the mistake from earlier was not caught and this will lead to CI checks succeeding despite format errors.

**Reproduction Full**
1. Create a new folder
2. Create a new project(put sln and csproj in same folder)
3. Install Paket and dotnet-format
4. Run `dotnet paket init`
5. Register `Microsoft.Extensions.DependencyInjection` to Paket, this can be done by adding "`Microsoft.Extensions.DependencyInjection`" into paket.references and "`nuget Microsoft.Extensions.DependencyInjection`" paket.dependencies.
6. Run `dotnet paket install`
7. Delete `.paket`. Simulate a mistake
8. Run `dotnet format [PROJECT].sln`

**Output**
```
Formatting code files in workspace 'C:\Users\[USER]\[REPO_PATH]\MissingRef.sln'.
Warnings were encountered while loading the workspace. Set the verbosity option to the 'diagnostic' level to log warnings.
Format complete in 2134ms.
```

**Diagnostic Output**
```
The dotnet format version is '5.1.250801+4a851ea9707f87d381166c2fc2b2d4b58a10a222'.
The dotnet runtime version is '5.0.7'.
The dotnet CLI version is '5.0.301'.
Using MSBuild.exe located in 'C:\Program Files\dotnet\sdk\5.0.301\'.
Formatting code files in workspace 'C:\Users\[USER]\[REPO_PATH]\MissingRef.sln'.
Loading workspace.
Msbuild failed when processing the file 'C:\Users\[USER]\[REPO_PATH]\MissingRef.csproj' with message: The imported project "C:\Users\[USER]\[REPO_PATH]\.paket\Paket.Restore.targets" was not found. Confirm that the expression in the Import declaration ".paket\Paket.Restore.targets" is correct, and that the file exists on disk. C:\Users\[USER]\[REPO_PATH]\MissingRef.csproj
Complete in 1374ms.
Determining formattable files.
Complete in 7ms.
Running formatters.
Complete in 7ms.
Formatted 0 of 0 files.
Format complete in 1392ms.
```

Both outputs, despite failing to load then format the project, returned an unexpected zero exit code. I noticed this on a CI where it would continue "succeed" despite my local copy telling me there's format errors.