Ignore FileNotFoundError when removing the lock file

[xuhcc]

Related issue: #115

[cameronmaske]

@xuhcc Thanks for the quick PR on this.
My only concern is that we are silently suppressing this.
I wonder if it's worth adding a warning or log of some sort to indicate the lock was removed prematurely?
Else, users who encounter this will most likely just assume the lock isn't working.
Any thoughts?

[xuhcc]

I agree that some kind of warning certainly would be useful in this case, but I'm not sure how to implement it. It seems that celery-once doesn't currently employ the logging module.

[cameronmaske]

@xuhcc Yeh, fair point. Adding logging is a whole separate improvement and can be tackled later.

[eyeteajay]

Hello, do you know how much "later" it will be to merge & release this?
It's nearly 2 years later but I'm getting the same traceback as described in #115 with version 3.0.1