Added Image.WARN_POSSIBLE_FORMATS

[radarhere]

Resolves #7993. Alternative to #8033

If a user tries to open the image from that issue

from PIL import Image
Image.open("bug.png")

they will see

Traceback (most recent call last):
  File "demo.py", line 2, in <module>
    Image.open("bug.png")
  File "PIL/Image.py", line 3380, in open
    raise UnidentifiedImageError(msg)
PIL.UnidentifiedImageError: cannot identify image file 'bug.png'

If the user would like to investigate further, this PR adds "warn_possible_formats". It will show any errors raised during the checking of the formats as warnings.

from PIL import Image
Image.WARN_POSSIBLE_FORMATS = True
Image.open("bug.png")

giving

PIL/Image.py:3378: UserWarning: PNG opening failed. broken PNG file (bad header checksum in b'pHYs')
  warnings.warn(message)
PIL/Image.py:3378: UserWarning: IM opening failed. Syntax error in IM header: �PNG
  warnings.warn(message)
PIL/Image.py:3378: UserWarning: IMT opening failed. not identified by this driver
  warnings.warn(message)
PIL/Image.py:3378: UserWarning: IPTC opening failed. invalid IPTC/NAA file
  warnings.warn(message)
PIL/Image.py:3378: UserWarning: PCD opening failed. not a PCD file
  warnings.warn(message)
PIL/Image.py:3378: UserWarning: SPIDER opening failed. not a valid Spider file
  warnings.warn(message)
PIL/Image.py:3378: UserWarning: TGA opening failed. not a TGA file
  warnings.warn(message)
Traceback (most recent call last):
  File "demo.py", line 3, in <module>
    Image.open("bug.png", warn_possible_formats=True)
  File "PIL/Image.py", line 3380, in open
    raise UnidentifiedImageError(msg)
PIL.UnidentifiedImageError: cannot identify image file 'bug.png'