[errors] Greatly simplify common/errors package.

deploytool/cmd/luci_deploy/title.go

Index: deploytool/cmd/luci_deploy/title.go
diff --git a/deploytool/cmd/luci_deploy/title.go b/deploytool/cmd/luci_deploy/title.go
index f2e77dbd5ce90396a119cae52bade88d0b01dcac..082a1b39129a55620920bbc3371807e0ecdef12c 100644
--- a/deploytool/cmd/luci_deploy/title.go
+++ b/deploytool/cmd/luci_deploy/title.go
@@ -21,8 +21,7 @@ func (t title) validate() error {
 	idx := 0
 	for _, r := range t {
 		if !(unicode.IsLetter(r) || unicode.IsNumber(r) || r == '-') {
-			return errors.Reason("character at %(pos)d (%(char)c) is not permitted in a title").
-				D("pos", idx).D("char", r).Err()
+			return errors.Reason("character at %d (%c) is not permitted in a title", idx, r).Err()
 		}
 		idx++
 	}