A broken LESS resource is always re-compiled

See: https://github.com/xwiki/xwiki-platform/blob/xwiki-platform-7.2/xwiki-platform-core/xwiki-platform-lesscss/xwiki-platform-lesscss-default/src/main/java/org/xwiki/lesscss/internal/cache/AbstractCachedCompiler.java#L135

When the compilation of a LESS resource fails, the result is not stored in the cache. As a result, every user who reach the resource (that could be main skin file for instance) will trigger a LESS compilation, and very quickly xwiki become very very slow and so unusable.

Instead, we should store in the cache, the serialized exception as a CSS comment.