fix: ensure global diagram context is cleared if rendering fails

diagrams/__init__.py

@@ -205,7 +205,10 @@ class Diagram:
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):
+        try:
             self.render()
+        finally:
+            if os.path.exists(self.filename):
                 # Remove the graphviz file leaving only the image.
                 os.remove(self.filename)
             setdiagram(None)