Specialize Exceptions thrown by UpdateManager (PR in hand!)

[ewlloyd]

Thanks for a great product!

One thing, though... Wouldn't it be better if client applications could respond to the specific error conditions reported by the UpdateManager class without having to parse the messages and guess? Let me propose the use of specialized exception classes to replace the bare System.Exceptions thrown today:

using(var mgr = new UpdateManager(squirrelSource)) {
	try {
		await mgr.UpdateApp();
	} catch(SquirrelNoUpdateException ex) {		
		// TODO: Deal with missing Update.exe,
		//       including ignoring it if we're running from source
	} catch(SquirrelReleasesMissingException ex) {
		// TODO: Deal with missing Releases folder
	} catch(SquirrelReleasesCorruptException ex) {
		// TODO: Deal with empty/corrupt RELEASES file
	} // otherwise continue throwing
}

All three exceptions noted above would be derived from SquirrelBaseException, so you have exactly the precision you need when customizing your catch blocks.

As noted above, I've implemented this in my fork, and I'll have a PR to you shortly.