scripts: Fixed failed subprocess stderr, unconditionally forward

It looks like the failure case in our scripts' subprocess stderr
handling was not tested well during a fix to stderr blocking (a735bcd).

This code was attempting to print stderr only if an error occured, but
with stderr=None this just results in a NoneType TypeError.

In retrospect, completely hiding stderr is kind of shitty if a
subprocess fails, but it doesn't seem possible to read from both stdin
and stderr with Python's APIs without getting stuck when the stderr's
buffer is full.

It might be possible to work around this with either multithreading,
select calls, or a temp file, but I'm not sure slightly less verbose
scripts are worth the added complexity in every single subprocess call.

For now just reverting to unconditionally forwarding stderr from the
child process. This is the simplest/most robust option.

scripts/cov.py

@@ -272,16 +272,12 @@ def collect_cov(gcda_path, *,
         print(' '.join(shlex.quote(c) for c in cmd))
     proc = sp.Popen(cmd,
             stdout=sp.PIPE,
-            stderr=None if args.get('verbose') else sp.DEVNULL,
             universal_newlines=True,
             errors='replace',
             close_fds=False)
     cov = json.load(proc.stdout)
     proc.wait()
     if proc.returncode != 0:
-        if not args.get('verbose'):
-            for line in proc.stderr:
-                sys.stderr.write(line)
         raise sp.CalledProcessError(proc.returncode, proc.args)
 
     return cov