Metamodel ceylon/ceylon-compiler#1572: do not hang forever and die with a meaningful exception
RuntimeModelLoader ceylon/ceylon-compiler#1572: load the module if possible
FreeClassOrInterface ceylon/ceylon-compiler#1572: skip overloaded abstractions for now
Metamodel: load module dependencies even for bootstrap modules
Metamodel: report UnknownType reasons as exceptions

With proper messages rather than bad luck ;)