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.

fs/rc/rcserver/rcserver.go

@@ -20,7 +20,6 @@ import (
 
 	"github.com/rclone/rclone/fs/rc/webgui"
 
-	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/skratchdot/open-golang/open"
@@ -138,7 +137,7 @@ func (s *Server) Serve() error {
 	if s.files != nil {
 		openURL, err := url.Parse(s.URL())
 		if err != nil {
-			return errors.Wrap(err, "invalid serving URL")
+			return fmt.Errorf("invalid serving URL: %w", err)
 		}
 		// Add username, password into the URL if they are set
 		user, pass := s.opt.HTTPOptions.BasicUser, s.opt.HTTPOptions.BasicPass
@@ -211,7 +210,7 @@ func (s *Server) handler(w http.ResponseWriter, r *http.Request) {
 	case "GET", "HEAD":
 		s.handleGet(w, r, path)
 	default:
-		writeError(path, nil, w, errors.Errorf("method %q not allowed", r.Method), http.StatusMethodNotAllowed)
+		writeError(path, nil, w, fmt.Errorf("method %q not allowed", r.Method), http.StatusMethodNotAllowed)
 		return
 	}
 }
@@ -225,7 +224,7 @@ func (s *Server) handlePost(w http.ResponseWriter, r *http.Request, path string)
 		// Parse the POST and URL parameters into r.Form, for others r.Form will be empty value
 		err := r.ParseForm()
 		if err != nil {
-			writeError(path, nil, w, errors.Wrap(err, "failed to parse form/URL parameters"), http.StatusBadRequest)
+			writeError(path, nil, w, fmt.Errorf("failed to parse form/URL parameters: %w", err), http.StatusBadRequest)
 			return
 		}
 		values = r.Form
@@ -243,20 +242,20 @@ func (s *Server) handlePost(w http.ResponseWriter, r *http.Request, path string)
 	if contentType == "application/json" {
 		err := json.NewDecoder(r.Body).Decode(&in)
 		if err != nil {
-			writeError(path, in, w, errors.Wrap(err, "failed to read input JSON"), http.StatusBadRequest)
+			writeError(path, in, w, fmt.Errorf("failed to read input JSON: %w", err), http.StatusBadRequest)
 			return
 		}
 	}
 	// Find the call
 	call := rc.Calls.Get(path)
 	if call == nil {
-		writeError(path, in, w, errors.Errorf("couldn't find method %q", path), http.StatusNotFound)
+		writeError(path, in, w, fmt.Errorf("couldn't find method %q", path), http.StatusNotFound)
 		return
 	}
 
 	// Check to see if it requires authorisation
 	if !s.opt.NoAuth && call.AuthRequired && !s.UsingAuth() {
-		writeError(path, in, w, errors.Errorf("authentication must be set up on the rc server to use %q or the --rc-no-auth flag must be in use", path), http.StatusForbidden)
+		writeError(path, in, w, fmt.Errorf("authentication must be set up on the rc server to use %q or the --rc-no-auth flag must be in use", path), http.StatusForbidden)
 		return
 	}
 
@@ -317,14 +316,14 @@ func (s *Server) serveRoot(w http.ResponseWriter, r *http.Request) {
 func (s *Server) serveRemote(w http.ResponseWriter, r *http.Request, path string, fsName string) {
 	f, err := cache.Get(s.ctx, fsName)
 	if err != nil {
-		writeError(path, nil, w, errors.Wrap(err, "failed to make Fs"), http.StatusInternalServerError)
+		writeError(path, nil, w, fmt.Errorf("failed to make Fs: %w", err), http.StatusInternalServerError)
 		return
 	}
 	if path == "" || strings.HasSuffix(path, "/") {
 		path = strings.Trim(path, "/")
 		entries, err := list.DirSorted(r.Context(), f, false, path)
 		if err != nil {
-			writeError(path, nil, w, errors.Wrap(err, "failed to list directory"), http.StatusInternalServerError)
+			writeError(path, nil, w, fmt.Errorf("failed to list directory: %w", err), http.StatusInternalServerError)
 			return
 		}
 		// Make the entries for display
@@ -343,7 +342,7 @@ func (s *Server) serveRemote(w http.ResponseWriter, r *http.Request, path string
 		path = strings.Trim(path, "/")
 		o, err := f.NewObject(r.Context(), path)
 		if err != nil {
-			writeError(path, nil, w, errors.Wrap(err, "failed to find object"), http.StatusInternalServerError)
+			writeError(path, nil, w, fmt.Errorf("failed to find object: %w", err), http.StatusInternalServerError)
 			return
 		}
 		serve.Object(w, r, o)