Print out which image caused error

[leijurv]

Exception message on encountering a corrupted image used to be:

libpng error: IDAT: CRC error.04081, loss_B: 0.05761
Exception in thread Thread-3:
Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 914, in _bootstrap_inner
    self.run()
  File "/home/leijurv/Documents/faceswap/lib/utils.py", line 42, in run
    for item in self.generator:
  File "/home/leijurv/Documents/faceswap/lib/training_data.py", line 43, in minibatch
    rtn = numpy.float32([read_image(data[j]) for j in range(i,i+size)])
  File "/home/leijurv/Documents/faceswap/lib/training_data.py", line 43, in <listcomp>
    rtn = numpy.float32([read_image(data[j]) for j in range(i,i+size)])
  File "/home/leijurv/Documents/faceswap/lib/training_data.py", line 25, in read_image
    image = cv2.imread(fn) / 255.0
TypeError: unsupported operand type(s) for /: 'NoneType' and 'float'

Now, it prints out which image specifically caused the error:

libpng error: IDAT: CRC error.03255, loss_B: 0.04847
Exception in thread Thread-3:
Traceback (most recent call last):
  File "/home/leijurv/Documents/faceswap/lib/training_data.py", line 26, in read_image
    image = cv2.imread(fn) / 255.0
TypeError: unsupported operand type(s) for /: 'NoneType' and 'float'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 914, in _bootstrap_inner
    self.run()
  File "/home/leijurv/Documents/faceswap/lib/utils.py", line 42, in run
    for item in self.generator:
  File "/home/leijurv/Documents/faceswap/lib/training_data.py", line 46, in minibatch
    rtn = numpy.float32([read_image(data[j]) for j in range(i,i+size)])
  File "/home/leijurv/Documents/faceswap/lib/training_data.py", line 46, in <listcomp>
    rtn = numpy.float32([read_image(data[j]) for j in range(i,i+size)])
  File "/home/leijurv/Documents/faceswap/lib/training_data.py", line 28, in read_image
    raise Exception("Error while reading image",fn)
Exception: ('Error while reading image', '/home/leijurv/faceswap/data/B/batch5_3970.png')

[Clorr]

Thanks for the commit ;-)