New test helper function to check that we got the error we wanted

This simplifies a bunch of our tests and, again, reduces duplication
Honestly, this doesn't really make things easier to read for me. checkError has 5 arguments, and it's hard to figure out what's going on and the savings is all of 2 lines.

There's also a bug here. Line 223 will fail if err is nil with "cannot convert nil to type waddrmgr.ManagerError". If you add that check, it's actually not making anything shorter.