Added a convention that sentinel errors go near the top of the file, and applied it

internal/types/opportunities.go

@@ -5,6 +5,10 @@ import (
 	"time"
 )
 
+// ErrWonAndLost is returned by OpportunityMarks.Validate when both Won and
+// Lost are set.
+var ErrWonAndLost = errors.New("types: an opportunity cannot be both won and lost")
+
 // Opportunity is a procurement opportunity (an OCDS contracting process,
 // identified by UUID) as of its latest release, with its classifications,
 // releases and the user's marks. Fields tagged db:"-" are stored in their
@@ -51,10 +55,6 @@ type OpportunityReleaseTag struct {
 	Tag       string `db:"tag"        json:"tag"`
 }
 
-// ErrWonAndLost is returned by OpportunityMarks.Validate when both Won and
-// Lost are set.
-var ErrWonAndLost = errors.New("types: an opportunity cannot be both won and lost")
-
 // OpportunityMarks are the user's own marks on an opportunity, kept apart
 // from the fetched data so saving a newer release never changes them. The
 // marks are independent except that Won and Lost exclude each other (see