Improve error message for generic type constructors
Improve error message for generic type constructors

FrancescAlted/datashape,aterrel/datashape,quantopian/datashape,cowlicks/datashape,llllllllll/datashape,markflorisson/datashape,ContinuumIO/datashape,cpcloud/datashape,llllllllll/datashape,quantopian/datashape,FrancescAlted/datashape,cowlicks/datashape,aterrel/datashape,markflorisson/datashape,blaze/datashape,ContinuumIO/datashape,cpcloud/datashape,blaze/datashape