Add custom exceptions, simplify generation template

tests/test_exceptions.py

@@ -0,0 +1,39 @@
+from dataclasses import dataclass
+
+import pytest
+
+from megasniff import SchemaInflatorGenerator
+from megasniff.exceptions import MissingFieldException, FieldValidationException
+
+
+def test_missing_field():
+    @dataclass
+    class A:
+        a: int
+
+    infl = SchemaInflatorGenerator()
+    fn = infl.schema_to_generator(A)
+    with pytest.raises(MissingFieldException):
+        fn({})
+
+
+def test_null():
+    @dataclass
+    class A:
+        a: int
+
+    infl = SchemaInflatorGenerator()
+    fn = infl.schema_to_generator(A)
+    with pytest.raises(FieldValidationException):
+        fn({'a': None})
+
+
+def test_invalid_field():
+    @dataclass
+    class A:
+        a: float | int | None
+
+    infl = SchemaInflatorGenerator()
+    fn = infl.schema_to_generator(A)
+    with pytest.raises(FieldValidationException):
+        fn({'a': {}})