Add exception type and update xml docs.

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

• Adds new InvalidImageContentException type.
• Documents exceptions and adds guards on all loading mechanisms.
• Documents exceptions and adds guards on all saving mechanisms.
• Minor cleanup and performance tweaks.

Fixes. #1110

[codecov]

Codecov Report

Merging #1179 into master will increase coverage by 0.00%.
The diff coverage is 74.54%.

@@           Coverage Diff           @@
##           master    #1179   +/-   ##
=======================================
  Coverage   82.47%   82.47%           
=======================================
  Files         688      690    +2     
  Lines       29891    29916   +25     
  Branches     3378     3379    +1     
=======================================
+ Hits        24653    24674   +21     
- Misses       4538     4541    +3     
- Partials      700      701    +1     

Flag	Coverage Δ
#unittests	82.47% <74.54%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Bmp/BmpInfoHeader.cs	82.51% <0.00%> (ø)
src/ImageSharp/Formats/Gif/GifDecoderCore.cs	83.76% <0.00%> (ø)
src/ImageSharp/Formats/Gif/GifThrowHelper.cs	0.00% <0.00%> (ø)
...olorConverters/JpegColorConverter.FromYCbCrSimd.cs	86.79% <0.00%> (ø)
...harp/Formats/Jpeg/Components/Decoder/JFifMarker.cs	86.84% <0.00%> (ø)
src/ImageSharp/Formats/Tga/TgaDecoderCore.cs	91.26% <0.00%> (ø)
src/ImageSharp/Formats/Tga/TgaThrowHelper.cs	0.00% <0.00%> (ø)
...harp/Processing/Extensions/ProcessingExtensions.cs	100.00% <ø> (ø)
src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs	94.44% <20.00%> (ø)
src/ImageSharp/Formats/Png/PngThrowHelper.cs	16.66% <20.00%> (ø)
... and 12 more

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[antonfirsov]

Great job! Will have another one in a few days.

[antonfirsov]

Can decoders throw InvalidImageContentException in Identify?

[JimBobSquarePants]

Yeah, they can hit a bad value in the image header.

[JimBobSquarePants]

Great job! Will have another one in a few days.

Quick as possible if you can. I'm hoping to ship in less than a week.

[brianpopow]

@JimBobSquarePants there are still some places left with a TODO: use InvalidImageContentException here, if we decide to define it

For example

ImageSharp/src/ImageSharp/Formats/Gif/GifDecoder.cs

Line 43 in 5639bab

throw new ImageFormatException($"Can not decode image. Failed to allocate buffers for possibly degenerate dimensions: {dims.Width}x{dims.Height}.", ex);

.

Was this intentional or should we change those, too?

[JimBobSquarePants]

@brianpopow I missed that. Yeah we should update.

[brianpopow]

@brianpopow I missed that. Yeah we should update.

@JimBobSquarePants ok i will do that