session: create errors.go file

And move an error here which we plan to use across DB types.

Author: ellemouton

session/errors.go

@@ -0,0 +1,9 @@
+package session
+
+import "errors"
+
+var (
+	// ErrSessionNotFound is an error returned when we attempt to retrieve
+	// information about a session but it is not found.
+	ErrSessionNotFound = errors.New("session not found")
+)

session/kvdb_store.go

@@ -49,10 +49,6 @@ var (
 	// IDs associated with the given group ID.
 	sessionIDKey = []byte("session-id")
 
-	// ErrSessionNotFound is an error returned when we attempt to retrieve
-	// information about a session but it is not found.
-	ErrSessionNotFound = errors.New("session not found")
-
 	// ErrDBInitErr is returned when a bucket that we expect to have been
 	// set up during DB initialisation is not found.
 	ErrDBInitErr = errors.New("db did not initialise properly")