[eximius]

I'd expect 'on_error' to take a function as a callback, not a string. But yes, a better named function could be added

[skybrian]

Perhaps or_die, similar to Perl?

[sanderjd]

Yep, this is what I always wanted it to be! But maybe or_panic would be better. Naming things is hard.

[allan_s]

or_fail_with("") ?

[eternalban]

expect[ing_error]("could not open the file")

[rootlocus]

You're not expecting an error. On the contrary, you're expecting a value or throwing an error.

[eternalban]

You're expecting specific errors.