Add ChildProcessError and ProcessLookupError

pep-3151.txt

@@ -396,6 +396,12 @@ the list of errnos mapped to them, is submitted to discussion:
   as a generic timeout exception, replacing ``socket.timeout`` and also useful
   for other types of timeout (for example in Lock.acquire())
 
+* ``ChildProcessError``: operation on a child process failed (ECHILD);
+  this is raised mainly by the wait() family of functions.
+
+* ``ProcessLookupError``: the given process (as identified by, e.g., its
+  process id) doesn't exist (ESRCH).
+
 In addition, the following exception class is proposed for inclusion:
 
 * ``ConnectionError``: a base class for ``ConnectionAbortedError``,
@@ -407,6 +413,7 @@ sub-hierarchy (OSError, assuming `Step 1`_ is accepted in full) is not
 shown::
 
     +-- BlockingIOError        EAGAIN, EALREADY, EWOULDBLOCK, EINPROGRESS
+    +-- ChildProcessError                                          ECHILD
     +-- ConnectionError
         +-- BrokenPipeError                              EPIPE, ESHUTDOWN
         +-- ConnectionAbortedError                           ECONNABORTED
@@ -418,6 +425,7 @@ shown::
     +-- IsADirectoryError                                          EISDIR
     +-- NotADirectoryError                                        ENOTDIR
     +-- PermissionError                                     EACCES, EPERM
+    +-- ProcessLookupError                                          ESRCH
     +-- TimeoutError                                            ETIMEDOUT
 
 Naming