pygrass/grid: provide at least basic error message

the error is visible in the lib/python/pygrass/modules/testsuite/test_doctest.py test


git-svn-id: https://svn.osgeo.org/grass/grass/trunk@62012 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/python/pygrass/modules/grid/grid.py

@@ -540,7 +540,7 @@ class GridModule(object):
             result = pool.map_async(cmd_exe, self.get_works())
             result.wait()
             if not result.successful():
-                raise RuntimeError
+                raise RuntimeError(_("Execution of subprocesses was not successful"))
 
         if patch:
             if self.move: