Improve error handling

src/error.rs

 #[derive(Debug, Clone)]
-pub struct OpossumError;
\ No newline at end of file
+pub enum OpossumError {
+  OpticScenery(String),
+  OpticGroup(String),
+  Other(String)
+}
\ No newline at end of file

src/nodes/node_group.rs

@@ -31,15 +31,15 @@ impl NodeGroup {
         target_node: NodeIndex,
     ) -> Result<EdgeIndex> {
         if self.g.node_weight(src_node).is_none() {
-            return Err(OpossumError);
+            return Err(OpossumError::OpticGroup("source node with gievn index does not exist".into()));
         }
         if self.g.node_weight(target_node).is_none() {
-            return Err(OpossumError);
+            return Err(OpossumError::OpticGroup("target node with given index does not exist".into()));
         }
         let edge_index = self.g.add_edge(src_node, target_node, ());
         if is_cyclic_directed(&self.g) {
             self.g.remove_edge(edge_index);
-            return Err(OpossumError);
+            return Err(OpossumError::OpticGroup("connecting the given nodes would form a loop".into()));
         }
         Ok(edge_index)
     }

src/optic_scenery.rs

@@ -45,15 +45,15 @@ impl OpticScenery {
         target_node: NodeIndex,
     ) -> Result<EdgeIndex> {
         if self.g.node_weight(src_node).is_none() {
-            return Err(OpossumError);
+            return Err(OpossumError::OpticScenery("source node with gievn index does not exist".into()));
         }
         if self.g.node_weight(target_node).is_none() {
-            return Err(OpossumError);
+            return Err(OpossumError::OpticScenery("target node with given index does not exist".into()));
         }
         let edge_index = self.g.add_edge(src_node, target_node, ());
         if is_cyclic_directed(&self.g) {
             self.g.remove_edge(edge_index);
-            return Err(OpossumError);
+            return Err(OpossumError::OpticScenery("connecting the given nodes would form a loop".into()));
         }
         Ok(edge_index)
     }