append pango code to message about color spec requiring manual fix

The commit message has the relevant details.

I consider this commit to be something of a stopgap. In the long term, there are a lot of improvements to be made to pangoize, such as:

* It only works on a message= key, inside a [message] tag. There are other places where markup is used; at the least, pangoize should probably check all translatables.
* It does not work when there is more than one markup symbol, for example for both large text and bold.
* Other stuff.

For now, this is a modest improvement; let's not make perfect the enemy of better.