fix errors

flax/errors.py

@@ -588,6 +588,46 @@ def __call__(self, x):
   def __init__(self):
     super().__init__('Can only call init, init_with_output or apply methods on an instance of the Module class, not the Module class itself')
 
+class IncorrectPostInitOverrideError(FlaxError):
+  """
+  This error occurs when you overrode `.__post_init__()` without calling `super().__post_init__()`.
+  For example, the error will be raised when trying to run this code::
+
+    from flax import linen as nn
+    import jax.numpy as jnp
+    import jax
+    class A(nn.Module):
+      x: float
+      def __post_init__(self):
+        self.x_square = self.x ** 2
+        # super().__post_init__() <-- forgot to add this line
+      @nn.compact
+      def __call__(self, input):
+        return input + 3
+
+    r = A(x=3)
+    r.init(jax.random.PRNGKey(2), jnp.ones(3))
+  """
+  def __init__(self):
+    super().__init__('Overrode `.__post_init__()` without calling `super().__post_init__()`')
+
+class DescriptorAttributeError(FlaxError):
+  """
+  This error occurs when you are trying to access a property that is accessing a non-existent attribute.
+  For example, the error will be raised when trying to run this code::
+
+    class Foo(nn.Module):
+        @property
+        def prop(self):
+            return self.non_existent_field # ERROR!
+        def __call__(self, x):
+            return self.prop
+
+    foo = Foo()
+    variables = foo.init(jax.random.PRNGKey(0), jnp.ones(shape=(1, 8)))
+  """
+  def __init__(self):
+    super().__init__('Trying to access a property that is accessing a non-existent attribute.')
 
 class InvalidCheckpointError(FlaxError):
   """