Added slightly more informative error message
Merge branch 'master' of https://github.com/cwi-swat/rascal
Merge branch 'master' of https://github.com/cwi-swat/rascal
Merge branch 'master' of https://github.com/cwi-swat/rascal
Merge branch 'master' of https://github.com/cwi-swat/rascal
Merge branch 'master' of https://github.com/cwi-swat/rascal
Merge branch 'master' of https://github.com/cwi-swat/rascal
Modified alias logic to ensure item always added, resolves #504