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/synchronize.py

@@ -270,13 +270,16 @@ class Condition(object):
 
     def notify(self, n=1):
         assert self._lock._semlock._is_mine(), 'lock is not owned'
-        assert not self._wait_semaphore.acquire(False)
+        assert not self._wait_semaphore.acquire(
+            False), ('notify: Should not have been able to acquire'
+                     + '_wait_semaphore')
 
         # to take account of timeouts since last notify*() we subtract
         # woken_count from sleeping_count and rezero woken_count
         while self._woken_count.acquire(False):
             res = self._sleeping_count.acquire(False)
-            assert res
+            assert res, ('notify: Bug in sleeping_count.acquire'
+                         + '- res should not be False')
 
         sleepers = 0
         while sleepers < n and self._sleeping_count.acquire(False):