Slightly improve a type-checking error message.
Slightly improve a type-checking error message.

diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,ihc/futhark,mrakgr/futhark,HIPERFIT/futhark,CulpaBS/wbBach,ihc/futhark,diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark,mrakgr/futhark,ihc/futhark,HIPERFIT/futhark,mrakgr/futhark