Added artiq exception example

examples/exception.py

@@ -0,0 +1,20 @@
+from artiq.experiment import *
+
+class ExceptionDemo(EnvExperiment):
+    def build(self):
+        self.setattr_device("core")
+        self.setattr_device("led0")
+
+    @kernel
+    def run(self):
+        self.core.reset()
+        print("OK!")
+        try:
+            try:
+                raise Exception
+            except ValueError as e:
+                print("re-raise")
+                raise e
+        except:
+            print("error")
+