Don't discard exception instance data in debug mode

If the debugger's monkey-patched raise was passed an instantiated
exception, the #exception method was called with a potentially nil
argument, causing its instance data to be thrown away. This hides
potentially useful information from the user.

Fix it by allowing instantiated exceptions to be reraised unharmed.

Fixes Homebrew/homebrew#17622.

Library/Homebrew/debrew.rb

@@ -137,10 +137,18 @@ module RaisePlus
 
   def raise(*args)
     exception = case
-      when args.size == 0 then ($!.nil? ? RuntimeError.exception : $!)
-      when (args.size == 1 and args[0].is_a?(String)) then RuntimeError.exception(args[0])
-      else args[0].exception(args[1]) # this does the right thing if args[1] is missing
-    end
+                when args.size == 0
+                  $!.nil? ? RuntimeError.exception : $!
+                when args.size == 1 && args[0].is_a?(String)
+                  RuntimeError.exception(args[0])
+                when args.size == 2 && args[0].is_a?(Exception)
+                  args[0].exception(args[1])
+                when args[0].is_a?(Class) && args[0].ancestors.include?(Exception)
+                  args[0].exception(args[1])
+                else
+                  args[0]
+                end
+
     # passing something other than a String or Exception is illegal, but if someone does it anyway,
     # that object won't have backtrace or continuation methods. in that case, let's pass it on to
     # the original raise, which will reject it