Improve exception message when loading an empty assembly

[svick]

Based on dotnet/roslyn#39470, a relatively common issue when working with dynamically generated assemblies is that, due to a bug in your code, you attempt to load an empty assembly (e.g., you call Assembly.Load(Stream) with a Stream of Length 0). When this happens, you get a correct, but not particularly helpful exception:

System.BadImageFormatException: Bad IL format.

Would it make sense to improve the exception message, to make it more clear what the issue is?

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