Nicer error when problem in install_requires

Instead of:

    $ python setup.py egg_info
    error in adminweb setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers

We now have the more helpful:

    $ python setup.py egg_info
    error in adminweb setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers.
    Error: Expected version spec in smsdk.authsvc>=0.0.8,2.0 at 2.0

It took me longer than it should to find the problem with the old error
message. The new error message would've helped greatly.

setuptools/dist.py

@@ -122,11 +122,15 @@ def check_requirements(dist, attr, value):
     """Verify that install_requires is a valid requirements list"""
     try:
         list(pkg_resources.parse_requirements(value))
-    except (TypeError,ValueError):
+    except (TypeError, ValueError) as e:
         raise DistutilsSetupError(
             "%r must be a string or list of strings "
-            "containing valid project/version requirement specifiers" % (attr,)
+            "containing valid project/version requirement specifiers.\n"
+            "Error: %s"
+            % (attr, ' '.join(e.args))
         )
+
+
 def check_entry_points(dist, attr, value):
     """Verify that entry_points map is parseable"""
     try: