Remove github.com/pkg/errors and replace with std library version

This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.

This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.

backend/cache/storage_memory.go

@@ -4,12 +4,12 @@
 package cache
 
 import (
+	"fmt"
 	"strconv"
 	"strings"
 	"time"
 
 	cache "github.com/patrickmn/go-cache"
-	"github.com/pkg/errors"
 	"github.com/rclone/rclone/fs"
 )
 
@@ -53,7 +53,7 @@ func (m *Memory) GetChunk(cachedObject *Object, offset int64) ([]byte, error) {
 		return data, nil
 	}
 
-	return nil, errors.Errorf("couldn't get cached object data at offset %v", offset)
+	return nil, fmt.Errorf("couldn't get cached object data at offset %v", offset)
 }
 
 // AddChunk adds a new chunk of a cached object