dotnet nbench with missing parameters fail gracefully

[heynickc]

If a performance test assembly isn't provided as a command line argument to dotnet nbench, should give the user the opportunity to quickly fix the mistake rather than unhandled exception like:

Unhandled Exception: System.ArgumentException: Please provide at least one test file.
Parameter name: files
   at NBench.Sdk.TestPackage..ctor(IEnumerable`1 files, IEnumerable`1 include, IEnumerable`1 exclude, Boolean concurrent)

Currently pops up a window saying "dotnet has stopped working..."

Could stand to simply display a message to "please enter a test assembly".