Deprecated skin designated to replace non-existant skin

In forrest.build.xml, a skin-aliasing occurs to replace skins which no longer exist by an alternative, and a warning is issued. Similarly, a warning is issued when a deprecated skin is chosen. It can happen that a deprecated skin is designated to replace a no-longer-existing skin: this should be avoided to prevent confusion. Maintenance should ensure that only non-deprecated (i.e. supported) skins are designated replacements each time the build file is updated or skins are deprecated.