[build] Report detailed errors when unable to find a usable mkisofs

[build] Report detailed errors when unable to find a usable mkisofs

As of commit 7c3d186 ("[build] Check that mkisofs equivalent supports
the required options"), we may refuse to use a mkisofs equivalent if
it does not support the options required to produce the requested
output file.

This can result in confusing error messages since the user is unaware
of the reason for which the installed mkisofs or genisoimage has been
rejected.

Fix by explicitly reporting the reason why each possible mkisofs
equivalent could not be used.

Signed-off-by: Michael Brown <mcb30@ipxe.org>