Python3 try/except handling improvements. Add Walker exception and use

that: fixes erroneous uncompyle success message on parse error.

test/simple_source/exception/05_try_except.py

@@ -1,6 +1,13 @@
+def handle(module):
+    try:
+        module = 1
+    except ImportError as exc:
+        module = exc
+    return module
+
 try:
     pass
 except ImportError as exc:
     pass
 finally:
-    del exc
+    y = 1