soften xml formation element

app/scodoc/sco_formations.py

@@ -180,8 +180,14 @@ def formation_import_xml(doc: str, import_tags=True):
         log("formation_import_xml: invalid XML data")
         raise ScoValueError("Fichier XML invalide")
 
-    f = dom.getElementsByTagName("formation")[0]  # or dom.documentElement
-    D = sco_xml.xml_to_dicts(f)
+    try:
+        f = dom.getElementsByTagName("formation")[0]  # or dom.documentElement
+        D = sco_xml.xml_to_dicts(f)
+    except:
+        raise ScoValueError(
+            "Le document xml ne correspond pas à un programme BUT. (élément 'formation' inexistant "
+            "par exemple)."
+        )
     assert D[0] == "formation"
     F = D[1]
     # F_quoted = F.copy()