Caught (general) exceptions not rethrown everywhere

[ehuelsmann]

The code base uses exceptions derived from ConditionThrowable (Go, Throw, Return) to cause non-local transfers of control.

A special exception (ThreadDestroyed) has been derived to cause a jump to the outer-most function in a LispThread. (This doesn't work in the main thread, which isn't enclosed by a function in LispThread like those in other threads.)

Some spots in our code seem to discard other exceptions than the ones mentioned above. That's probably wrong in most cases.

[ehuelsmann]

All occurrances of "catch (Throwable" have been reviewed; most have been removed, some have been made more specific.