ReadCondHandle: print error if no extended event context is set
If no event context is set, EventContext::getExtension
will throw a
std::bad_any_cast
exception, which sometimes cannot easily be traced
back to its origin (see e.g. ATR-19412). This typically will happen if
one tries to use a ReadCondHandle
before the first event. To make the
diagnostics easier, print a more explicit error message and re-throw.