Fix: Exists() should always return false when an error happenning and FLW parsing enhancement

[jasonjoo2010]

This PR solved 2 small issues togethor:

1. Newer zookeeper (tested with 3.6.x) outputs a little different in 4-letter-word response.
2. Exists() acts different to ExistsW()

Description

FLW

Newer server will response different on latency element:

Latency min/avg/max: 0/0.9167/8

Which seems like some float numbers.

So I adjust the regex expression to pass it.

Exists()

Exists() will return true with an error except ErrNoNode:

error happened	result	error returned
nil	true	nil
ErrNoNode	false	nil
Any other	~true~(incorrect)	error

In addition ExistsW() acts correctly.