Add Last Raised Exception In The Call Stack as a ruby til.

ruby/last-raised-exception-in-the-call-stack.md

@@ -0,0 +1,18 @@
+# Last Raised Exception In The Call Stack
+
+In Ruby, the `$!` global variable contains the last exception that was
+raised in the current call stack. This makes it trivial to check what error
+is being rescued even if it hasn't been captured in a local variable.
+
+```ruby
+class MyError < StandardError; end
+
+def do_stuff
+  raise MyError
+rescue
+  puts "rescuing #{$!}"
+end
+
+do_stuff
+#=> rescuing MyError
+```