Using ErrFailed on both ctx.Done and when retries are exhausted

[Moulik245]

If the context is canceled while trying to acquire a lock, ErrFailed is returned, which has the message "redsync: failed to acquire lock" :

redsync/mutex.go

Line 98 in b292c9f

return ErrFailed

but ErrFailed is also returned when all retries have been exhausted:

redsync/mutex.go

Line 133 in b292c9f

return ErrFailed

This makes it harder to tell whether the lock was not acquired due to context being canceled or retries being exhausted. I think it would be better if these situations returned different error messages.