[errors] Greatly simplify common/errors package.

client/isolate/utils.go

 // Copyright 2015 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 isolate
 
 import (
-        "fmt"
         "log"
         "path"
         "runtime"
         "strings"
 
         "github.com/luci/luci-go/common/errors"
 )
 
 // TODO(tandrii): Remove this hacky stuff.
 // hacky stuff for faster debugging.
 func assertNoError(err error) {
         if err == nil {
                 return
         }
-        log.Panic(errors.RenderStack(fmt.Errorf("assertion failed due to error: %s", err)).ToLines())
+        log.Panic(errors.RenderStack(errors.Annotate(err, "assertion failed").Err()))
 }
 
 func assert(condition bool, info ...interface{}) {
         if condition {
                 return
         }
         if len(info) == 0 {
-                log.Panic(errors.RenderStack(errors.New("assertion failed")).ToLines())
+                log.Panic(errors.RenderStack(errors.New("assertion failed")))
         } else if format, ok := info[0].(string); ok {
-                log.Panic(errors.RenderStack(fmt.Errorf("assertion failed: "+format, info[1:]...)).ToLines())
+                log.Panic(errors.RenderStack(errors.Reason("assertion failed: "+format, info[1:]...).Err()))
         }
 }
 
 // uniqueMergeSortedStrings merges two sorted sets of string (as slices) and removes duplicates.
 func uniqueMergeSortedStrings(ls, rs []string) []string {
         varSet := make([]string, len(ls)+len(rs))
         for i := 0; ; i++ {
                 if len(ls) == 0 {
                         rs, ls = ls, rs
                 }
@@ skipping 88 matching lines @@
                 }
                 return string(buf), nil
         }
         return targ[t0:], nil
 }
 
 // IsWindows returns True when running on the best OS there is.
 func IsWindows() bool {
         return runtime.GOOS == "windows"
 }