- rename errocodes → gray_errors
- restructure the errors into a `error_spec` type
- make the list of errors easily extensible
- rewrite the `print_errn`, `print_errhcd` (now `print_err_raytracing`,
`print_err_ecrh_cd`) subroutine to handle arbitrary errors
- add functions to easily manipulate errors
(`raise_error`, `has_error`, `is_critical`)
- remove print statements from quadpack
- log all errors to stderr using the logger module
