commands.rb: Extend 'display_api_exception' to print error messages

So you can see what GitHub is complaining about.  For example,

  $ hub merge-button https://github.com/defunkt/hub/pull/9999999999
  Error pushing merge button: Not Found (HTTP 404)
  #<Net::HTTPNotFound:0x7f4cf845c670>
  {"message":"Not Found"}

This was useful for debugging, when I originally thought that the
whole PUT body was optional if you didn't want to specify
'commit_message'.  In that case, I got:

  $ hub merge-button wking/email-issue-reply-testing#2
  Error pushing merge button: Bad Request (HTTP 400)
  #<Net::HTTPBadRequest:0x7f944f75a450>
  {"message":"Body should be a JSON Hash"}

Which is a lot more helpful than just "Bad Request".  It would be nice
if the 'Net::HTTP*' object hash was not printed, but I lack sufficient
Ruby-foo to know how to do that.

lib/hub/commands.rb

@@ -400,7 +400,7 @@ def merge_button(args)
       args.executable = 'echo'
       args.replace ['merged', '%s#%s' % [project.name_with_owner, pull_id]]
     rescue GitHubAPI::Exceptions
-      display_api_exception("pushing merge button", $!.response)
+      display_api_exception("pushing merge button", $!.response, $!.data)
       exit 1
     end
 
@@ -1048,8 +1048,11 @@ def expand_alias(cmd)
       end
     end
 
-    def display_api_exception(action, response)
+    def display_api_exception(action, response, message=nil)
       $stderr.puts "Error #{action}: #{response.message.strip} (HTTP #{response.status})"
+      if message
+        $stderr.puts message
+      end
       if 422 == response.status and response.error_message?
         # display validation errors
         msg = response.error_message