doc: add try-catch-finally example file

doc/sample/Exception.im

@@ -0,0 +1,12 @@
+try
+    x = 0
+    --v = Int parse:'342'
+catch (#err:number_format, err) in
+    x = 1
+    --cout put:'Cannot parse integer string ({err})'
+catch (_, err) in
+    x = 2
+    --cout put:'Unknown error occurred ({err})'
+finally
+    x = 3
+end