Display exit code when nonzero.

Brew fails if a tool (make, or whatever) doesn't return an exit code
of 0. This patch displays the non-zero code on failure, so we can
better diagnose what caused the build to fail (or if we need to add
that exit code as exception 'success code'.)

Library/Homebrew/formula.rb

@@ -184,6 +184,7 @@ protected
         end
       end
       unless $? == 0
+        puts "Exit code: #{$?}"
         puts out
         raise
       end

Library/Homebrew/utils.rb

@@ -69,7 +69,10 @@ def safe_system cmd, *args
   # CTRL-C interrupt to us too, so execution continues, but the exit code os
   # still 2 so we raise our own interrupt
   raise Interrupt, cmd if $?.termsig == 2
-  raise ExecutionError.new(cmd, args) unless exec_success and $?.success?
+  unless exec_success and $?.success?
+    puts "Exit code: #{$?}"
+    raise ExecutionError.new(cmd, args)
+  end 
 end
 
 def curl url, *args