Remove exception-driven programming from EventProvider

[cvonelm]

The EventProvider makes good use of exception-driven programming of the form:

try
{
open/lookup/whatever_event()
}
catch
{
//event does not exist is not openable etc.

try_next_source()
}

besides try-catch cascades being ugly as all balls, I burned myself while trying to implement pmu-events support (getting events via the same interface perf uses so we match perf events 1:1) with some shenanigans that are probably half constructed objects (?).

Anyways, I think replacing the exception part with std::optional or just returning some kind of EventDescription that is tagged as "invalid" would be better. (The latter is afaik already the direction we are moving in, in #340 )