Document exceptions correctly

[antonfirsov]

• Image.Load overloads are marked as throwing UnknownImageFormatException however when the image content is incorrect, we throw ImageFormatException instead. We should consider defining, utilizing and documenting a new InvalidImageContentException for these cases. Alternatively we can just make the xmldocs to refer to ImageFormatException, as an easier solution, but having a separate ImageFormatException subclass seems more correct to me.
• In Image.FromFile.cs and Image.FromBytes.cs we do not document the above exceptions at all.
• ImageProcessingException is not documented on the API surface. I would suggest to add it to: Mutate, Clone and IImageProcessor<TPixel>.Execute, however I haven't analyzed the call hierarchy whether it is the correct thing to do.
• Review our API if there are any other missing exception docs.

If we decide to indroduce InvalidImageContentException, that part should be probably done for RC1, since it is a breaking change.

[antonfirsov]

Also noticed that many Image.Load overloads and other public methods are not validating their input at all.