Feature request: make the ErrorWindow internationalizable

[michaelknigge]

It would be great if the labels of the ErrorWindow could be customizable. This would make it possible to use language specific labels.

[michaelknigge]

I've created a PR for this, see #17

[mlopezFC]

Thanks @michaelknigge for the contribution (both the issue and the PR)!
As you already might know we are reviewing the changes, will let you know when we have news.
Regards!

[javier-godoy]

Version 3.1.0 have been published in Vaadin directory.