server.py: Fail gracefully when serializing/deserializing a tree.

Currently, t.explore() will dump the tree to /tmp/*tid*.pickle, and use it
if to "undo" an ultrametric visualization (by reloading the original tree
from disk). Sometimes the trees cannot be pickled, and this was not considered.

Now at least it fails gracefully. But I think we would be better by not dumping
the tree at all, and not allowing the visualization of trees to modify the
tree itself and pretend that it is not. Editing the tree should be explicit.

ete4/smartview/gui/server.py

@@ -732,11 +732,20 @@ def retrieve_tree_data(tid):
 
     It retrieves all that from a previously saved pickle file in /tmp."""
     # Called when tree has been deleted from memory.
-    tree_data = pickle.load(open(f'/tmp/{tid}.pickle', 'rb'))
+    try:
+        tree_data = pickle.load(open(f'/tmp/{tid}.pickle', 'rb'))
+    except (FileNotFoundError, EOFError, pickle.UnpicklingError) as e:
+        print(f'Tree {tid} cannot be recovered from disk. Loading placeholder.')
+        tree_data = TreeData()
+        tree_data.name = 'Placeholder tree'
+        tree_data.tree = Tree('(could,not,load,tree);')
+        ops.update_sizes_all(tree_data.tree)
+
     tree_data.style = copy_style(TreeStyle())
     tree_data.layouts = retrieve_layouts(tree_data.layouts)
     tree_data.active = drawer_module.get_empty_active()
     tree_data.timer = time()  # to track if it is active
+
     return tree_data
 
 
@@ -1445,7 +1454,12 @@ def write_tree_data():
         data.style = None  # since it can't be pickled
         data.layouts = layouts  # same
         data.active = None  # same
-        pickle.dump(data, open(f'/tmp/{tid}.pickle', 'wb'))
+        try:
+            pickle.dump(data, open(f'/tmp/{tid}.pickle', 'wb'))
+        except (pickle.PicklingError, PermissionError) as e:
+            print(f'Tree {tid} not saved to file.')
+            # So changing to ultrametric and back will not work,
+            # because it is done by re-reading from the dumped file.
     thr_write = Thread(daemon=True, target=write_tree_data)  # so we are not delayed
     thr_write.start()                                   # by big trees