Allow calling tf.Error, tf.Errorf in seperate goroutines

[Robert Pająk]

// A task ends when its action function returns or calls any of the methods
// FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf.
//
// All methods must be called only from the goroutine running the
// Action function.

should be changed (and implemented) to:

// A task ends when its action function returns or calls any of the methods
// FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf.
// Those methods must be called only from the goroutine running the action function.

// The other reporting methods, such as the variations of Log and Error, may be called simultaneously from multiple goroutines.