mtf2json: improved error messages for fluff section

mtf2json/mtf2json.py

@@ -679,11 +679,14 @@ def __add_fluff(key: str, value: str, fluff_section: Dict[str, Union[str, List[s
     # the key is already in the fluff section
     # -> it's a subsection
     if key in fluff_section:
-        subkey, subvalue = value.split(':', 1)
+        try:
+            subkey, subvalue = value.split(':', 1)
+        except Exception as e:
+            raise ConversionError(f"Key '{key}' already exists in the fluff section but value is missing the ':' delimiter: {e}")
         if isinstance(fluff_section[key], dict):
             cast(dict, fluff_section[key])[subkey.lower()] = subvalue.strip()
         else:
-            raise ConversionError(f"Fluff key entry '{key}' is not a dictionary!")
+            raise ConversionError(f"Tried to add '{subkey}:{subvalue}' to fluff section '{key}', but '{key}' is not a dictionary!")
     # the key is new
     else:
         # value contains a subkey