feat: skip middleware, plugins, and exception reporting when exceptions are disabled

[stympy]

This PR makes the exceptions.enabled configuration option work a little better by doing the following:

• Skip loading ErrorNotifier and associated middleware
• Skip hooking into Sidekiq, etc. to report exceptions
• Abort sending any notifications when calling Honeybadger.notify

[joshuap]

I vaguely remember using the exceptions.enabled config option in the past, but we don't list it in the docs, and it looks like the only place it's currently used is in the sinatra integration. Is that right? 😅

I'm OK with this PR. I think using the requirement block syntax I mentioned where possible would make some of the plugins cleaner, but obviously we can't do that for plugins that also do Insights things (unless we separate them).

@stympy are you sure this covers all the exceptions features in the plugins, though?

[joshuap]

@roelbondoc did you review any of the other plugins?

[roelbondoc]

@roelbondoc did you review any of the other plugins?

I went through all the plugins and this covers everything except for the ErrorSubscriber in the rails plugin.

However, there is a early return based on the config in Honeybadger::Agent#notify method that will prevent any calls so this would cover anything we missed including custom invocations in an application.