Add explicit/helpful error message about how to solve the multiprocessing 'Permission denied' issue.

src/commands.py

@@ -101,7 +101,17 @@ def process(f, *args, **kwargs):
         except Exception as e:
             raise e
     
-    q = multiprocessing.Queue()
+    try:
+        q = multiprocessing.Queue()
+    except OSError:
+        log.error('Using multiprocessing.Queue raised an OSError.\n'
+                'This is probably caused by your system denying semaphore\n'
+                'usage. You should run these two commands:\n'
+                '\tsudo rmdir /dev/shm\n'
+                '\tsudo ln -Tsf /{run,dev}/shm\n'
+                '(See https://github.com/travis-ci/travis-core/issues/187\n'
+                'for more informations about this bug.)\n')
+        raise
     def newf(f, q, *args, **kwargs):
         if resource:
             rsrc = resource.RLIMIT_DATA