Throw ValueError when discriminator cannot be found (#88)

* Throw ValueError when discriminator cannot be found

This provides the receiver with more details about the Commercetools
response

* Fix formatting

Co-authored-by: Tim Leguijt <t.leguijt@labdigital.nl>

Author: tleguijt

src/commercetools/helpers.py

@@ -66,7 +66,7 @@ def __init__(
         only=None,
         discriminator_field=None,
         discriminator_schemas=None,
-        **kwargs
+        **kwargs,
     ):
         # Raise error if only or exclude is passed as string, not list of strings
         if only is not None and not is_collection(only):
@@ -149,7 +149,13 @@ def _load(self, value, data, **kwargs):
             return None
 
         discriminator_value = value[self.discriminator_field[0]]
-        schema_name = self.discriminator_schemas[discriminator_value]
+        try:
+            schema_name = self.discriminator_schemas[discriminator_value]
+        except KeyError:
+            raise ValueError(
+                f"Could not find discriminator schema {discriminator_value} for field '{self.name}' ({value})"
+            )
+
         schema = self.get_schema(schema_name)
 
         try: