Make error handling consistent

[Zylatis]

As noted in current docs:

Not all internal errors are handled the same way. For example, if during the process of checking if rotation is required an error occurs, the default is to print a warning to stdout and not rotate. In contrast to this, if an error occurs during the actual rotation procedure, this error is bubbled up through error handling eventually returning as a std::io::Error to the caller. However probable future state will outsource all error handling logic to the caller of this library rather than making assumptions.

The question is: do we pass all errors up to the caller of write, which might be annoying for transient ones when checking if we should rotate, or do we add some config options to let people decide at compile time. Personally I think the former is safest, at least the user knows where they stand.