default: fix error handling in pipeCommand()

Traceback (most recent call last):
  ...
  File "slapos/recipe/build/__init__.py", line 191, in pipeCommand
    raise zc.buildout.UserError('Failed while running command:'
  File "slapos/recipe/build/__init__.py", line 192, in <genexpr>
    + ''.join('\n  ' + q for q in failed))
TypeError: cannot concatenate 'str' and 'tuple' objects

slapos/recipe/build/__init__.py

@@ -178,17 +178,17 @@ class Script(EnvironMixin):
       p = subprocess.Popen(command_list, *args, **kwargs)
       if previous is not None:
         previous.stdout.close()
-      subprocess_list.append((p, command_list))
-      run_list.append(' '.join(command_list))
+      command = ' '.join(command_list)
+      subprocess_list.append((p, command))
+      run_list.append(command)
       previous = p
-    self.logger.info('Running: %r' % ' | '.join(run_list))
-    failed = []
-    for q in reversed(subprocess_list):
-      if q[0].wait():
-        failed.append(q[1])
+    self.logger.info('Running: %s', ' | '.join(run_list))
+    failed = [command
+      for p, command in reversed(subprocess_list)
+      if p.wait()]
     if failed:
-      raise zc.buildout.UserError('Failed while running command:'
-        + ''.join('\n  ' + q for q in failed))
+      raise zc.buildout.UserError('Failed while running:\n  '
+        + '\n  '.join(failed))
 
   def copyTree(self, origin, destination, ignore_dir_list=None):
     """Recursively Copy directory.