Unify scriptlet execution calling conventions and error handling

Unify scriptlet execution calling conventions and error handling
- Split external scriptlet execution to separate function, called
  identically with internal (ie lua) scriptlet run
- runScript() is now just a dispatcher which calls the lower level script
  runner and decides whether the scriptlet failure is fatal or not.
- Only %prein and %preun scriptlet failures are considered fatal
  regardless of the type of failure, as they prevent the install/erase
  from taking place at all. Other failures might or might not be fatal
  in terms of package functionality but rpm has no way of knowing whether
  that is the case, so we only report what we know for a fact:
  either the package was installed/erased or not, and either it had
  scriptlet failures (for which callbacks are issued) or it didn't.