raise custom exceptions when required/requested items are not found

setup.py

@@ -40,6 +40,8 @@ _LIB_IMAGING = (
 
 DEBUG = False
 
+class DependencyException(Exception): pass
+class DefaultDependencyException(Exception): pass
 
 def _dbg(s, tp=None):
     if DEBUG:
@@ -558,12 +560,8 @@ class pil_build_ext(build_ext):
         for f in feature:
             if not getattr(feature, f) and feature.require(f):
                 if f in ('jpeg', 'zlib'):
-                    raise ValueError(
-                        '%s is required unless explicitly disabled'
-                        ' using --disable-%s, aborting' % (f, f))
-                raise ValueError(
-                    '--enable-%s requested but %s not found, aborting.' %
-                    (f, f))
+                    raise DefaultDependencyException(f)
+                raise DependencyException(f)
             
         #
         # core library
@@ -758,14 +756,14 @@ class pil_build_ext(build_ext):
 def debug_build():
     return hasattr(sys, 'gettotalrefcount')
 
-
-setup(name=NAME,
+try:
+    setup(name=NAME,
           version=PILLOW_VERSION,
           description='Python Imaging Library (Fork)',
           long_description=_read('README.rst').decode('utf-8'),
           author='Alex Clark (Fork Author)',
           author_email='aclark@aclark.net',
-      url='https://python-pillow.org',
+          url='http://python-pillow.org',
           classifiers=[
               "Development Status :: 6 - Mature",
               "Topic :: Multimedia :: Graphics",
@@ -793,3 +791,25 @@ setup(name=NAME,
           keywords=["Imaging", ],
           license='Standard PIL License',
           zip_safe=not debug_build(), )
+except DefaultDependencyException as err:
+    msg = """
+
+The headers or library files could not be found for %s,
+a required dependency when compiling Pillow from source.
+
+Please see the install instructions at:
+   http://pillow.readthedocs.io/en/latest/installation.html
+
+""" % (str(err))
+    sys.stderr.write(msg)
+    raise DefaultDependencyException(msg)
+except DependencyException as err:
+    msg = """
+
+The headers or library files could not be found for %s,
+which was requested by the option flag --enable-%s
+
+""" % (str(err), str(err))
+    sys.stderr.write(msg)
+    raise DependencyException(msg)
+