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