Improve ErrorWindow styles

[javier-godoy]

• The message below the title should have word-break: break-all.

• That element should have a css class so that it can be targeted for styling (with a sane CSS selector)

• That element is a span (which is an inline element). In production mode, there is a div (which is a block element) inside of it.

[mlopezFC]

@javier-godoy Do you think that something like this would be fine?