Make some methods open to allow custom Throwable format

[ichenhe]

The default handling of Throwable is to print the full stack, which may be too large.

Actually, I don't think prepareLog should append the throwable to the message so that the subclass can decide what to do with them.

But in order not to break the behavior, could you make prepareLog() or getStackTraceString() open and protecte? Then I can change the format.