Refactor exception handling to build map of arbitrary data, and not just messages

[anderseknert]

In order to include things like location — or other arbitrary data — in our exceptions (as raised in #124) we'll need to refactor our exception functions to take a map of data rather than a string to build the exception message. Creating a separate issue for this as a refactor like this is going to be a pretty big undertaking, and feels out of scope for the original PR.