exec: use exit code 255 for exec failures, same paused error as runc

[kolyshkin]

1. exec: use exit code 255 for exec failures

   On success, crun exec exits with the exit code of the executed process. On failure of crun exec itself (e.g. no such container, no such executable), it exits with 1, and so the caller cannot tell it apart from the executed process exiting with 1.

   Use exit code of 255 for crun exec failures, like runc does since v1.1.0 (see runc exec: fail with exit code of 255 opencontainers/runc#3073), which is the same convention as used by e.g. ssh. Add a test case.

   Note that podman (via conmon) does not rely on the crun exec exit code, as it gets the error details from conmon.

2. exec: use the same paused container error as runc

   When the container is paused, crun exec fails with the container `ID` is paused, while runc uses cannot exec in a paused container. Use the latter (appending the container ID), as some tools and tests check for it.

Found by the "runc exec [exit codes]" and "runc exec should refuse a paused container" runc integration tests (see #2238).

[giuseppe]

LGTM