MySQL tweaks.

Firstly, I changed the method that ErrorStore uses to get the assembly path - as per the comments I made on pull request #49.

So - re: `LogError` in MySQL. I'm a bit stumped. MySQL.NET data connector doesn't support OUT params unless it's a sproc. So you can't fire an update that returns a GUID like you do.

I tried a few different things, and I've tweaked the code a little, here are the options:

* Use `ON DUPLICATE KEY UPDATE` to insert/update in one hit, but that wouldn't work with the minDate criteria, because it has to be based unique index on the table (so we'd have to include the rollup date as a column to make that work)

* Create a sproc. Would work, but feels wrong

* Wrap anything we're still concerned about in a transaction if that will help, which I'm not sure about.