Add a helpful message when tools are missing

While you can build the unit tests and run them without Visual Studio
2015 (or the build tools) installed, you need to have it installed to
run the program or build.cmd. Generate a helpful error when these
are missing.

Author: ellismg

build.cmd

@@ -4,6 +4,16 @@ SETLOCAL
 
 SET CACHED_NUGET=%LocalAppData%\NuGet\NuGet.exe
 SET SOLUTION_PATH=%~dp0src\CodeFormatter.sln
+SET VS2015_BUILD_TOOLS_PATH="%ProgramFiles(x86)%\MSBuild\14.0\bin\MSBuild.exe"
+
+IF NOT EXIST %VS2015_BUILD_TOOLS_PATH% (
+  echo In order to build or run this tool you need either Visual Studio 2015 Preview or
+  echo Microsoft Build Tools 2015 Preview tools installed.
+  echo.
+  echo Visit http://www.visualstudio.com/en-us/downloads/visual-studio-2015-downloads-vs 
+  echo to download either.
+  goto :eof
+)
 
 IF EXIST %CACHED_NUGET% goto restore
 echo Downloading latest version of NuGet.exe...