[errors] Greatly simplify common/errors package.

vpython/application/subcommand_delete.go

Index: vpython/application/subcommand_delete.go
diff --git a/vpython/application/subcommand_delete.go b/vpython/application/subcommand_delete.go
index 7f387d333ba14b5b11eae967c420881049f47d63..b2b611764026efb1f8aa9ba40d54d07cb21bfd54 100644
--- a/vpython/application/subcommand_delete.go
+++ b/vpython/application/subcommand_delete.go
@@ -70,9 +70,7 @@ func (cr *deleteCommandRun) Run(app subcommands.Application, args []string, env
 		}
 
 		if failures > 0 {
-			return errors.Reason("failed to delete %(count)d environment(s)").
-				D("count", failures).
-				Err()
+			return errors.Reason("failed to delete %d environment(s)", failures).Err()
 		}
 		return nil
 	})