erp5_core: return instead of raise StopIteration

@luke report an error when do search

and we have the error log below:

  Module script, line 50, in Base_getSummaryAsHTML
   - <PythonScript at /nexedi/Base_getSummaryAsHTML used for /nexedi/web_page_module/xxx>
   - Line 50
    found = context.Base_showFoundText()
  Module Products.ERP5Type.patches.PythonScript, line 181, in __call__
    return self._orig_bindAndExec(args, kw, None)
  Module Shared.DC.Scripts.Bindings, line 372, in _bindAndExec
    return self._exec(bound_data, args, kw)
  Module Products.PythonScripts.PythonScript, line 354, in _exec
    result = function(*args, **kw)
  Module script, line 52, in Base_showFoundText
   - <PythonScript at /nexedi/Base_showFoundText used for /nexedi/web_page_module/xxx>
   - Line 52
    found_text_fragments = context.Base_getExcerptText(
  Module Products.ERP5Type.patches.ExternalMethod, line 127, in __call__
    return _f[0](*args, **kw)
  Module erp5.component.extension.erp5_version.DocumentExtraction, line 117, in getExcerptText
    return [p for p in generateParts(context,text,sw,tags,trail,maxlines)]
  Module erp5.component.extension.erp5_version.DocumentExtraction, line 117, in <listcomp>
    return [p for p in generateParts(context,text,sw,tags,trail,maxlines)]
RuntimeError: generator raised StopIteration

when check more, i found the behavior of raise StopIteration in generator change in python2&python3

My proposition is to use return instead of raise to have the same behavior

Test	Python 2.7	Python 3.7+
raise StopIteration in generator	Normal stop	RuntimeError
return in generator	Normal stop	Normal stop

a simple test to produce the behavior

def gen():
    yield 1
    raise StopIteration
    yield 2

print(list(gen()))

/cc @jerome @romain