[s1879] Improve MMap File loading related log output

Log error of missing map files only if MMaps are active for this map
Log missing tiles only in debug mode (without LOG_FILTER_MAP_LOADING)

Thanks to qwertz1000 for pointing to problems
Signed-off-by: Schmoozerd <schmoozerd@cmangos>

(based on commit [12346] - 06b5800)