Small logging improvements for addon loading

Adds back the "Loading file" message when a file is loaded Adds individual file errors for things loaded with W_InitMultipleFiles (-file param) Errors for a given file will now appear between "Loading file" and the "Added file" or "not adding file" messages, making troubleshooting easier

To easily test these changes, take any community pk3 addon that works, open it like a zip file and copy or make some superfluous changes outside of slade, then change it back to pk3 and try loading it with the -file parameter. It also makes it easy to attribute other types of errors that happen during W_InitFile