Fixed crash when folders contain msbuild-style projects.

[sharpninja]

Rebased as requested.

[TAGC]

Cheers mate.

I'd prefer for now if it handled msbuild style projects in the following way:

• Gracefully fails with ExitFailure if you're trying to update a specific project, and that project happens to be msbuild-style.
• Skips msbuild-style projects when running the program in recursive mode in a directory tree containing several projects, some of which may be msbuild-style.

I've added failing integration tests in the commit I pushed to your develop branch. I'd appreciate if you did the following:

• Replace ExampleMsBuildStyleCsprojFile with the contents of a minimal msbuild-style csproj file, since I presume you have one on hand that prompted you to open this PR.
• Update the program logic so those two integration tests pass.

I noticed that the build succeeded even though my integration tests failed, so I'll be pushing some changes to this repo's develop branch to fix my AppVeyor config so they fail. You'll need to rebase on those changes. (Edit) I've done this, no worries.

[sharpninja]

It would make sense to go ahead and extend functionality to including updating AssemblyInfo.cs and .nuspec files.

[TAGC]

I guess, but that would require more work. Going down that route, I'd first like to make dotnet-setversion gracefully fail for (or skip) old-style projects, release that as v2.0.1, then after that functionality to bump versions in other types of files could be implemented as a minor-level release.

[TAGC]

I'm going to close this for now, but let me know if you want it re-opened.