SimpleLibrary.open gives misleading exception

[Jonathan Gibbons]

Consider: 
     javac -L dir args

If dir exists but is not a valid library, I get a FileNotFoundException 
from SimpleLibrary.open regarding the internal %jigsaw-library file, as 
shows up here:

error: module library not found: 
/w/jjg/work/jigsaw/jigsaw.lt/play/modules/%jigsaw-library (No such file 
or directory)

It would be better to get a message saying the path given to 
SimpleLibrary.open is not a valid module library. It would be even 
better to get a specific subtype of IOException, so that javac does not 
have to rely on a non-localized exception detail message.

-- Jon