Refine exception wrapping mechanism

[dakusui]

Right now Checks.rethrow wraps and rethrow checked exceptions from lower levels.
Instead we shoud introduce Checks.wrap checked exception and 'return' wrapping runtime exception and callers should be responsible for throwing returned runtime exception.
By this we will not need to suppress around warnings caused by the rethrow method (because it makes java compiler not understand the method is aborted at the points where the method is called)