fix: added missing import for sources_deploy.py example

Author: ashley-b

examples/extensions/deployers/sources/custom_deployer_sources.rst

@@ -65,6 +65,7 @@ The **source_deploy.py** file has the following code:
 .. code-block:: python
     :caption: **sources_deploy.py**
 
+    from conan.errors import ConanException
     from conan.tools.files import copy
     import os