added some more sanity checking to SliceAdaptor::fetch_by_region - cannot create slice on non-existant or toplevel CoordSystem
added some more sanity checking to SliceAdaptor::fetch_by_region - cannot create slice on non-existant or toplevel CoordSystem

mjg17/ensembl,mjg17/ensembl,danstaines/ensembl,muffato/ensembl,Ensembl/ensembl,willmclaren/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,muffato/ensembl,Ensembl/ensembl,danstaines/ensembl,muffato/ensembl,muffato/ensembl,at7/ensembl,james-monkeyshines/ensembl,danstaines/ensembl,at7/ensembl,james-monkeyshines/ensembl,at7/ensembl,willmclaren/ensembl,Ensembl/ensembl,mjg17/ensembl,Ensembl/ensembl,at7/ensembl