src/error: add forgotten event_graph error

src/error.rs

@@ -427,6 +427,12 @@ pub enum Error {
     #[error("contract execution error")]
     ContractExecError(u64),
 
+    // ====================
+    // Event Graph errors
+    // ====================
+    #[error("Event is not found in tree: {0}")]
+    EventNotFound(String),
+
     // ====================
     // Miscellaneous errors
     // ====================