Fix error propagation from memoryview-returning functions

A code like
|cdef double[:] foo():
|    raise AttributeError('dummy')

generated C code like this:
|  __pyx_L1_error:;
|  (...)
|  __pyx_r.memview = NULL;
|  __Pyx_AddTraceback("view_return_errors.foo", __pyx_clineno, __pyx_lineno, __pyx_filename);
|  __pyx_L0:;
|  if (unlikely(!__pyx_r.memview)) {
|    PyErr_SetString(PyExc_TypeError,"Memoryview return value is not initialized");
|  }
|  __Pyx_RefNannyFinishContext();
|  return __pyx_r;
|}

Which is incorrect in error case, because we set __pyx_r.memview to NULL and
then we test it, so that the PyErr_SetString() is always called in the error
case, which swallows previously-set error. (it also swallowed the traceback,
which I don't understand)

A fix is to jump to return_from_error_cleanup label in case return type is
memoryview, as it is currently done for the case when buffers are present.

A testcase that fauils w/out this fix applied is included, too.