Database updates (#144)

* start moving some database stuff around

* continue moving db stuff around

* more fiddling

* more updates

* and some more

* and yet more

* i broke SOMETHING but what, it's a mystery

* tidy up

* vendor ttlcache

* use ttlcache

* fix up some tests

* rename some stuff

* little reminder

* some more updates

internal/db/pg/util.go

@@ -0,0 +1,25 @@
+package pg
+
+import (
+	"strings"
+
+	"github.com/go-pg/pg/v10"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+)
+
+// processErrorResponse parses the given error and returns an appropriate DBError.
+func processErrorResponse(err error) db.Error {
+	switch err {
+	case nil:
+		return nil
+	case pg.ErrNoRows:
+		return db.ErrNoEntries
+	case pg.ErrMultiRows:
+		return db.ErrMultipleEntries
+	default:
+		if strings.Contains(err.Error(), "duplicate key value violates unique constraint") {
+			return db.ErrAlreadyExists
+		}
+		return err
+	}
+}