Silence exceptions in _repr_jpeg_ and _repr_png_

[mtreinish]

Resolves #7283

In 10.0.0 a _repr_jpeg_ implementation was added to the Image object to enable the use of display_jpeg() in IPython environments. However, in some cases the implementation of this method could result in an exception being raised while trying to generate the jpeg data. The best example is if the image data is in an RGBA format as a result of the object being created by opening a PNG file. In this case trying to save the Image object as a jpeg will error because the jpeg format can't represent the transparency in the alpha channel. This results in an exception being raised in the IPython/Jupyter context when outputting the image object. However, in cases like this IPython allows the repr methods to return None to indicate there is no representation in that format available. [1] This commit updates the _repr_png_ and _repr_jpeg_ methods to catch any exception that might be raised while trying to generate the image data. If an exception is raised we treat that as not being able to generate image data in that format and return None instead.

Related to #7259

[1] https://ipython.readthedocs.io/en/stable/config/integrating.html#custom-methods

Changes proposed in this pull request:

• Handle any exceptions raised by generating image data in Image._repr_png_ and return None instead of raising to users.
• Handle any exceptions raised by generating image data in Image._repr_jpeg_ and return None instead of raising to users.

[radarhere]

I've created mtreinish#1 with a suggestion to simplify the code.