Add explicit exceptions during command calls / handling

[hikinggrass]

Allow exceptions thrown in cmd handlers to be propagated to the calling module

This allows the caller to handle certain error conditions more gracefully (and could be extended for shutdown handling during cmd callc in the future as well)

A module could handle these new exceptions in the following way:

try {
    req->call_something();
} catch (const Everest::HandlerException& e) {
    EVLOG_error << "got a HandlerException: " << e.what();
} catch (const Everest::CmdTimeout& e) {
    EVLOG_error << "got a CmdTimeout: " << e.what();
} catch (const Everest::CmdError& e) {
    EVLOG_error << "got a CmdError: " << e.what();
}

HandlerExceptions and CmdTimeout inherit from CmdError so a caller could also just catch this.

(naming still up for debate)

[hikinggrass]

ErrorType might be a bit vague and could lead to confusions with the "error framework"