Pointing to a non-existent `csproj` produces a confusing error message

[shilangyu]

When trying to generate contracts using the project flag and then pointing to a non-existent file produces an exception that does indeed mention lack of the file, but also mentions other problems which might be misleading. Example:

 Generator script failed with: Cannot compile project or generate contracts: The project file could not be loaded. Could not load file or assembly 'System.Security.Permissions, Version=4.0.3.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'. The system cannot find the file specified.
   /home/shilangyu/coding/work/contractsgenerator-dart/packages/leancode_contracts_generator/contractsgenerator/examples/project/aggregated/Combined/Combined.csproj
 At
    at Microsoft.Build.Shared.ProjectFileErrorUtilities.VerifyThrowInvalidProjectFile(Boolean condition, String errorSubCategoryResourceName, BuildEventFileInfo projectFile, Exception innerException, String resourceName, Object[] args)
    at Microsoft.Build.Shared.ProjectFileErrorUtilities.ThrowInvalidProjectFile(BuildEventFileInfo projectFile, Exception innerException, String resourceName, Object[] args)
    at Microsoft.Build.Construction.ProjectRootElement.CreateProjectFromPath(String projectFile, ProjectRootElementCacheBase projectRootElementCache, Boolean preserveFormatting)
    at Microsoft.Build.Construction.ProjectRootElement.<>c.<OpenProjectOrSolution>b__208_0(String path, ProjectRootElementCacheBase cache)
    at Microsoft.Build.Evaluation.ProjectRootElementCache.Get(String projectFile, OpenProjectRootElement openProjectRootElement, Boolean isExplicitlyLoaded, Nullable`1 preserveFormatting)
    at Microsoft.Build.Construction.ProjectRootElement.OpenProjectOrSolution(String fullPath, IDictionary`2 globalProperties, String toolsVersion, ProjectRootElementCacheBase projectRootElementCache, Boolean isExplicitlyLoaded)
    at Microsoft.Build.Evaluation.ProjectCollection.LoadProject(String fileName, IDictionary`2 globalProperties, String toolsVersion)
    at LeanCode.ContractsGenerator.Compilation.MSBuild.MSBuildHelper.RestoreProjects(IReadOnlyCollection`1 projectPaths, IDictionary`2 globalProperties) in /home/runner/work/contractsgenerator/contractsgenerator/src/LeanCode.ContractsGenerator/Compilation/MSBuild/MSBuildHelper.cs:line 69
    at LeanCode.ContractsGenerator.Compilation.ProjectLoader.LoadProjectsAsync(IEnumerable`1 projectPaths) in /home/runner/work/contractsgenerator/contractsgenerator/src/LeanCode.ContractsGenerator/Compilation/ProjectLoader.cs:line 24
    at LeanCode.ContractsGenerator.Compilation.ContractsCompiler.CompileProjectsAsync(IEnumerable`1 projectPaths) in /home/runner/work/contractsgenerator/contractsgenerator/src/LeanCode.ContractsGenerator/Compilation/ContractsCompiler.cs:line 68
    at LeanCode.ContractsGenerator.Program.HandleProjectAsync(ProjectOptions p) in /home/runner/work/contractsgenerator/contractsgenerator/src/LeanCode.ContractsGenerator/Program.cs:line 114
    at LeanCode.ContractsGenerator.Program.Main(String[] args) in /home/runner/work/contractsgenerator/contractsgenerator/src/LeanCode.ContractsGenerator/Program.cs:line 61