Issue 388: HandlingResult.getException() throws exp if none thrown.

When calling HandlingResult.getException() during a unit test and no
exception was thrown, the method throws a
HandlerExceptionNotThrownException instead of returning null.
Issue 388 - Pass expected exception type to HandlingResult.exception(c)

Rename method from getException() to exception(Class<T> type). Methods
throws exception if the type throw is not the expected type passed to
the method.
Issue 388: Throw exception when accessing handle results after excep.
Merge pull request #473 from johnrengelman/handling-result-excep

Issue 388: Improved error handling in HandlingResult.
Javadoc for exec harness.
Don't “overload” getter for this property.
Fix perf app.
Don't “overload” getter.
Convert Rx Observables to Ratpack Promises. (Issue 468)

Supports testing observables by converting them to promises and using
the existing ExecHarness methods. (Issue 384)