Clarify how guards handle exceptions

[stephan]

The language definition doesn't mention whether exceptions are allowed in a guard predicate expression. The compiler currently treats guards as boolean methods with no declared exceptions, which means:

• a guard predicate cannot throw a checked exception
• an unchecked exception within a guard will unexpectedly interrupt the control flow including the original base behaviour.

The first case causes some inconvenience if a guard wants to use a method that declares a checked exception, because the guard itself cannot syntactically catch the exception -> need to introduce a helper method.

The second case is unfortunate because the base behaviour should not be affected by an exception thrown in a guard -- the only effect of a guard should be to filter when callin bindings fire.

[stephan]

Resolved by

• writing new OTJLD §5.4(c) (r19722)
• provide new tests 9.1.9-otjld-exception-in-predicate-1, 9.1.9-otjld-exception-in-predicate-2
• implementation (also in r19722):
  • generated code now catches all exceptions thrown from a predicate expression and returns false instead
  • compiler has new configurable diagnostic ("exceptioninguard") to report about this situation