MNT better message for pillow import error (scikit-learn#14027)

* better import error message for pillow * pillow -> Pillow * link to pillow page

Author: NicolasHug

sklearn/externals/_pilutil.py

@@ -66,6 +66,14 @@
 __all__ = ['bytescale', 'imread', 'imsave', 'fromimage', 'toimage', 'imresize']
 
 
+PILLOW_ERROR_MESSAGE = (
+ "The Python Imaging Library (PIL) is required to load data "
+ "from jpeg files. Please refer to "
+ "https://pillow.readthedocs.io/en/stable/installation.html "
+ "for installing PIL."
+)
+
+
 def bytescale(data, cmin=None, cmax=None, high=255, low=0):
  """
  Byte scales an array (image).
@@ -194,8 +202,7 @@ def imread(name, flatten=False, mode=None):
 
  """
  if not pillow_installed:
- raise ImportError("The Python Imaging Library (PIL) "
- "is required to load data from jpeg files")
+ raise ImportError(PILLOW_ERROR_MESSAGE)
 
  im = Image.open(name)
  return fromimage(im, flatten=flatten, mode=mode)
@@ -281,8 +288,7 @@ def fromimage(im, flatten=False, mode=None):
 
  """
  if not pillow_installed:
- raise ImportError("The Python Imaging Library (PIL) "
- "is required to load data from jpeg files")
+ raise ImportError(PILLOW_ERROR_MESSAGE)
 
  if not Image.isImageType(im):
  raise TypeError("Input is not a PIL image.")
@@ -349,8 +355,7 @@ def toimage(arr, high=255, low=0, cmin=None, cmax=None, pal=None,
 
  """
  if not pillow_installed:
- raise ImportError("The Python Imaging Library (PIL) "
- "is required to load data from jpeg files")
+ raise ImportError(PILLOW_ERROR_MESSAGE)
 
  data = asarray(arr)
  if iscomplexobj(data):