clarify: ch-run: error: can't execve(2): ... Exec format error

[reidpr]

Running a container on the wrong architecture yields an error something like:

ch-run[121666]: error: can't execve(2): /sbin/ldconfig: Exec format error (ch_core.c:549 8)

This error is clear if you have the right obscure nerd knowledge, but most people don’t, and we get questions periodically about it. This issue is to clarify the error. Options include:

1. FAQ entry and/or hint (could trigger hint on errno == ENOEXEC).
2. Check the target executable somehow.