Replace bytestring with unsafe_string

[giordano]

Fix #63.

[ludvigak]

I think bytestring should be replaced with unsafe_string, not string.

Also, it might make more sense to put all the changes to custom_error_handler mentioned in #84 in one PR, i.e. also change to @warn and supply arguments to the exceptions, since this PR by itself is not yet functional.

[giordano]

I think bytestring should be replaced with unsafe_string, not string.

I think you're right, I've updated the PR.

Also, it might make more sense to put all the changes to custom_error_handler mentioned in #84 in one PR, i.e. also change to @warn and supply arguments to the exceptions, since this PR by itself is not yet functional.

Well, this PR has nothing to do with the custom_error_handler and fixes a reported issue, it just happened that I found out the issues with that function while trying to reproduce #63.

[ludvigak]

You're right of course. I was just thinking that the example in #63 still fails, though there are currently three separate reasons why it fails. Didn't mean to complain when you're the one actually trying to fix thing =)