Prevent invoked expectations from being modified after they are invoked.

I paired with @mrgilman on this. I'm happy to carry the pull request over the line based on feedback.
It's going to be faster to use `\` to continue the message rather than putting it in an array and joining it.

Also, "Whilst" is such a british-ism (I can't think if any time I've ever heard an American use that word).  Mind using "while" instead so that this doesn't sound old-fashioned to American users?

FWIW, I found this discussion of "whilst" vs "while" interesting:

http://www.grammar-monster.com/easily_confused/while_whilst.htm

Also, "mocked method" might be confusing if they are using `allow`.  Maybe this?

``` ruby
error_message = "The message expectation for #{orig_object.inspect}.#{message} has already been invoked " \
  "and cannot be modified further (e.g. using `#{calling_customization}`). All message expectation " \
  "customizations must be applied before it is used for the first time."
```