Utility: don't print useless output from resource compiler.

Print something only if compilation failed.
Utility: don't *ever* return nullptr on error in Resource::getRaw().

When the group is overriden and the file cannot be opened just fall
back to compiled-in one. We are asserting if the file does not exist,
so the user isn't expecting this at all.
Utility: fix misleading Resource::get() documentation.