Fix: Once a groovy script has a compiletime error, it's never compiled again.

This includes changes:
- In execute(), re-raise existing errors only if the scrips has not
  changed.
- For this to work: reset ``compileTime`` prior to compiling the script.
  (Otherwise canUseOldCompiledScript() could not be run in execute() due
  to it integrated check-delay.)
- Clear ``errorsInScript`` prior to compiling. (Instead of not compiling,
  at all if ``errorsInScript are set.)
- Same changes for GenericScript.