Fix excessive retries missing --max-duration timeout - fixes #4504

This change checks the context whenever rclone might retry, and
doesn't retry if the current context has an error.

This fixes the pathological behaviour of `--max-duration` refusing to
exit because all the context deadline exceeded errors were being
retried.

This unfortunately meant changing the shouldRetry logic in every
backend and doing a lot of context propagation.

See: https://forum.rclone.org/t/add-flag-to-exit-immediately-when-max-duration-reached/22723

fs/fserrors/error_test.go

@@ -1,6 +1,7 @@
 package fserrors
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"net"
@@ -170,3 +171,21 @@ func TestRetryAfter(t *testing.T) {
 	assert.True(t, IsRetryAfterError(err))
 	assert.Contains(t, e.Error(), "try again after")
 }
+
+func TestContextError(t *testing.T) {
+	var err = io.EOF
+	ctx, cancel := context.WithCancel(context.Background())
+
+	assert.False(t, ContextError(ctx, &err))
+	assert.Equal(t, io.EOF, err)
+
+	cancel()
+
+	assert.True(t, ContextError(ctx, &err))
+	assert.Equal(t, io.EOF, err)
+
+	err = nil
+
+	assert.True(t, ContextError(ctx, &err))
+	assert.Equal(t, context.Canceled, err)
+}