Improve handling of hostcall errors

[tschneidereit]

In a lot of places, we currently turn a hostcall error code into a JS exception containing the error code and the C++ source location of the hostcall. That's very rarely helpful in debugging JS code, and we should improve on it :)

[JakeChampion]

@tschneidereit I just came across the mapping of hostcall error codes within the fastly-shared crate and thought I'd have a go at contributing to this project -- I've started a draft pull-request with the error messages from the crate

[tschneidereit]

Reopening since this isn't yet fully addressed