Currently the tool prints problems right as it is checking the code
without an intermediate error representation. However for various reasons
it would be beneficial to have an intermediate error type:
- It makes the code cleaner, having all errors in one place
- It allows printing the error in different ways, e.g. for a future
--json mode
This commit prepares for an incremental refactoring for an intermediate
error/problem representation. Most notable is that we want to be able to collect
multiple errors/problems and not just exit on the first one.
We introduce the type alias CheckResult and CheckError (later renamed to
NixpkgsProblem), where CheckError allows collecting multiple
CheckErrors using the utility function flatten_check_results (later
renamed to check_result::sequence)
The write_check_result function is only temporarily introduced to help
refactoring, it's removed again in later commits.
