FMod - IMF import crash fix

Today I received a bug report from an Abiathar user. He found that importing a certain IMF song using the Song Mappings dialog caused Abiathar to crash. Investigating, we found that just the specific IMF file was corrupt - its header specified far more notes than the file actually contained, so Abiathar unexpectedly hit the EOF when trying to load it all. So Abiathar didn't have so much an IMF handling problem as an error handling one, but obviously it's bad to bring down the entire application when confronted with invalid resources, so I added the appropriate error handling.