add base signal class

ormar/exceptions.py

@@ -1,28 +1,57 @@
 class AsyncOrmException(Exception):
+    """
+        Base ormar Exception
+    """
+
     pass
 
 
 class ModelDefinitionError(AsyncOrmException):
+    """
+        Raised for errors related to the model definition itself.
+        * setting @property_field on method with arguments other than func(self)
+        * defining a Field without required parameters
+        * defining a model with more than one primary_key
+        * defining a model without primary_key
+        * setting primary_key column as pydantic_only
+    """
+
     pass
 
 
 class ModelError(AsyncOrmException):
-    pass
+    """
+        Raised for initialization of model with non-existing field keyword.
+    """
 
-
-class ModelNotSet(AsyncOrmException):
     pass
 
 
 class NoMatch(AsyncOrmException):
+    """
+        Raised for database queries that has no matching result (empty result).
+    """
+
     pass
 
 
 class MultipleMatches(AsyncOrmException):
+    """
+        Raised for database queries that should return one row (i.e. get, first etc.)
+        but has multiple matching results in response.
+    """
+
     pass
 
 
 class QueryDefinitionError(AsyncOrmException):
+    """
+        Raised for errors in query definition.
+        * using contains or icontains filter with instance of the Model
+        * using Queryset.update() without filter and setting each flag to True
+        * using Queryset.delete() without filter and setting each flag to True
+    """
+
     pass
 
 
@@ -31,4 +60,17 @@ class RelationshipInstanceError(AsyncOrmException):
 
 
 class ModelPersistenceError(AsyncOrmException):
+    """
+        Raised for update of models without primary_key set (cannot retrieve from db)
+        or for saving a model with relation to unsaved model (cannot extract fk value).
+    """
+
+    pass
+
+
+class SignalDefinitionError(AsyncOrmException):
+    """
+        Raised when non callable receiver is passed as signal callback.
+    """
+
     pass