Fix up default callback, so that it can be set for each package as well as for all packages.
Callbacks now have 3 levels of control, which may be a bit too complex, default callbacks might have to go for debugging reasons. Either that or some kind of origination information should be appended to the error array describing which callbacks were called, so debugging is at least possible
fix 1444: PEAR_ErrorStack->push() needs to validate $msg and $code as string/int
add error level checking to hasErrors() and getErrors().
BC break!!:
staticGetErrors adds the parameter $level between $purge and $merge, as it will be as common (or more common) as $purge.
