Rename Guard class to Throw

[dlemstra]

The current code uses a Guard class but this should be refactored to Throw. An example implementation can be found here: https://github.com/dlemstra/Magick.NET/blob/master/Source/Magick.NET/Shared/Helpers/Throw.cs.

[beebop-fr]

The provided example link is 404.

[dlemstra]

This link should work: https://github.com/dlemstra/Magick.NET/blob/44802ce86f37d6986690fc56bb9221262144890b/src/Shared/Throw.cs.