fix: a panic error occurs in the func WritePoint when dbserver has been down #79

[njbestway]

This PR is to fix 2 problems:

1. It will panic in the func WritePoint when All DBServers are Down. The resp maybe nil when any Err ocurrs.
2. It should check whether the callback func is nil or not indeed. Just making func call using the callback directly doesn't seem friendly.It proves to be slightly confusing.

[hezhangjian]

Suggested change

	return err
	return nil

I prefer to return nil here since we decide to handle this error in callback.
And it's clear that anyone who didn't pass callback will miss error, callback should be passed in production.

[hezhangjian]

I changed it to return nil and merged this PR because the previous code decided to handle this error in the callback. I think it's quite clear that anyone who doesn't pass a callback will miss the error. The callback should be passed in production. Feel free to open an issue or continue in #79
to discuss it

[njbestway]

I changed it to return nil and merged this PR because the previous code decided to handle this error in the callback. I think it's quite clear that anyone who doesn't pass a callback will miss the error. The callback should be passed in production. Feel free to open an issue or continue in #79 to discuss it

Agree! Nobody wants to dealwith both the err func Passed and the err Returned.