get_data_mod: fix error message when module not found (pythondata modules are named only with "-" and not "_").

litex/__init__.py

@@ -11,6 +11,8 @@ def get_data_mod(data_type, data_name):
         dm = l['dm']
         return dm
     except ImportError as e:
+        data_type = data_type.replace("_", "-")
+        data_name = data_name.replace("_", "-")
         raise ImportError("""\
 pythondata-{dt}-{dn} module not installed! Unable to use {dn} {dt}.
 {e}