[errors] Greatly simplify common/errors package.

luci_config/server/cfgclient/resolver.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package cfgclient
 
 import (
         "github.com/luci/luci-go/common/errors"
         "github.com/luci/luci-go/luci_config/server/cfgclient/backend"
 )
@@ skipping 32 matching lines @@
         // Format returns the FormatterRegistry key and associated data for this
         // Resolver.
         //
         // An empty format represents no Formatter, meaning that this Resolver only
         // supports the raw config service result.
         Format() backend.FormatSpec
 }
 
 func assertEmptyFormat(it *backend.Item) error {
         if !it.FormatSpec.Unformatted() {
-                return errors.Reason("unknown format: %(format)q").D("format", it.FormatSpec.Formatter).Err()
+                return errors.Reason("unknown format: %q", it.FormatSpec.Formatter).Err()
         }
         return nil
 }
 
 // Bytes is a Resolver that resolves config data into a byte slice.
 func Bytes(out *[]byte) Resolver { return byteSliceResolver{out} }
 
 // BytesSlice is a MultiResolver that resolves condig data into a slice of byte
 // slices.
 func BytesSlice(out *[][]byte) MultiResolver { return multiByteSliceResolver{out} }
@@ skipping 65 matching lines @@
         }
 }
 
 func (r multiStringResolver) ResolveItemAt(i int, it *backend.Item) error {
         if err := assertEmptyFormat(it); err != nil {
                 return err
         }
         (*r.out)[i] = it.Content
         return nil
 }