Remove the subprocess "bad exception data" warning (formerly a print!)

all together and just include the repr of the data in the exception
itself instead of the useless string "Unknown".

This code path is unlikely to even be possible to take given the
nature of the pipe it gets subprocess data from.

Lib/subprocess.py

@@ -1412,11 +1412,10 @@ class Popen(object):
                     exception_name, hex_errno, err_msg = (
                             errpipe_data.split(b':', 2))
                 except ValueError:
-                    warnings.warn(RuntimeWarning(
-                            'Bad exception data: %r' % errpipe_data))
                     exception_name = b'RuntimeError'
                     hex_errno = b'0'
-                    err_msg = b'Unknown'
+                    err_msg = (b'Bad exception data from child: ' +
+                               repr(errpipe_data))
                 child_exception_type = getattr(
                         builtins, exception_name.decode('ascii'),
                         RuntimeError)

Misc/NEWS

@@ -83,6 +83,9 @@ Core and Builtins
 Library
 -------
 
+- Remove a bare print to stdout from the subprocess module that could have
+  happened if the child process wrote garbage to its pre-exec error pipe.
+
 - Issue #16327: The subprocess module no longer leaks file descriptors
   used for stdin/stdout/stderr pipes to the child when fork() fails.