Improve GuiActionRunner

At the end of GuiActionRunner there is the following code:
```java
if (caughtException instanceof RuntimeException) {
appendStackTraceInCurrentThreadToThrowable(caughtException, "execute");
throw (RuntimeException) caughtException;
}
if (caughtException instanceof Error) {
caughtException.fillInStackTrace();
throw (Error) caughtException;
}
```
I think it is not reasonable to fill in the stack trace for the Errors, throwing away the original stack trace: Errors should be treated the same as RuntimeExceptions. Or - if there is some reason for doing so - then at least don't throw away the stack trace of AssertionErrors, they are normal and expected during testing.