A better warning message when a file already exists

[pbalduino]

https://github.com/caelum/tubaina/blob/master/src/main/java/br/com/caelum/tubaina/resources/LatexResourceManipulator.java#L50-L54

I think it's not a good practice to log a warning using the text "Error" inside the message.

It's hard enough to find where the error happened and this message really doesn't help.

Anyway, I don't even think this could be considered a warning, once I'm using the same image in different chapters and I think there's no problem in it.