Guile exceptions

Error handling dominates most production programss. While we like to focus on the sunny day scenarios and domain models, in reality most of production code detects and handles the "alternate courses" caused by the myriad of error conditions. The quality of error handling and error messages often makes the difference between an enjoyable and a barely usable system. Good error messages make happy users and accelerate adoption. Rust's excellent compiler error messages, for example, play a large role in its suc