bpo-5001: More-informative multiprocessing error messages (#3079)

* Make error message more informative

Replace assertions in error-reporting code with more-informative version that doesn't cause confusion over where and what the error is.

* Additional clarification + get travis to check

* Change from SystemError to TypeError

As suggested in PR comment by @pitrou, changing from SystemError; TypeError appears appropriate.

* NEWS file installation; ACKS addition (will do my best to justify it by additional work)

* Making current AssertionErrors in multiprocessing more informative

* Blurb added re multiprocessing managers.py, queues.py cleanup

* Further multiprocessing cleanup - went through pool.py

* Fix two asserts in multiprocessing/util.py

* Most asserts in multiprocessing more informative

* Didn't save right version

* Further work on multiprocessing error messages

* Correct typo

* Correct typo v2

* Blasted colon... serves me right for trying to work on two things at once

* Simplify NEWS entry

* Update 2017-08-18-17-16-38.bpo-5001.gwnthq.rst

* Update 2017-08-18-17-16-38.bpo-5001.gwnthq.rst

OK, never mind.

* Corrected (thanks to pitrou) error messages for notify

* Remove extraneous backslash in docstring.

Lib/multiprocessing/reduction.py

@@ -165,7 +165,10 @@ else:
                 if len(cmsg_data) % a.itemsize != 0:
                     raise ValueError
                 a.frombytes(cmsg_data)
-                assert len(a) % 256 == msg[0]
+                if len(a) % 256 != msg[0]:
+                    raise AssertionError(
+                        "Len is {0:n} but msg[0] is {1!r}".format(
+                            len(a), msg[0]))
                 return list(a)
         except (ValueError, IndexError):
             pass