opam install fails with exitcode 5 instead of 20 when a package is unavailable

[kit-ty-kate]

Similar to #4493
Using opam 2.2.0~beta3 and ocaml/opam-repository#26072:

$ opam install conf-libssl.1
Error:  conf-libssl = 1: unmet availability conditions: 'os != "win32" | os-distribution = "cygwinports"'
$ echo $?
5

According to the documentation, exit code 5 means:

5 Not found. You requested something (package, version, repository, etc.) that couldn't be found.

Instead, one would expect exit code 20:

20 There is no solution to the user request. This can be caused by asking to install two incompatible packages, for example.

[kit-ty-kate]

The source of the issue is located in OpamSolution.check_availability

opam/src/client/opamSolution.ml

Line 192 in 9c7a256

OpamStd.Sys.exit_because `Not_found)

which seems to mix both concept of "unavailability because the package does not (longer) exist" and "unavailability because the package is not available on this platform"

Based on how OpamSwitchState.unavailable_reason is used everywhere else with the correct exit_reason, i strongly believe this is a bug.