Cleanup functions should return erors

[nejtr0n]

Is your feature request related to a problem? Please describe.

Cleanup function has bad signature without returning error now.
Basic usage of this functions to close connection or other resources.
So it should be better to return error imho.

Describe the solution you'd like

Change cleanup function signature from func() to func() error or io.Closer contract

Describe alternatives you've considered

Dunno how to handle cleanup errors now.