libdwfl: When the kernel is found, but not the modules warn, don't fail.

libdwfl: When the kernel is found, but not the modules warn, don't fail.

For -k the argp parser could fail to find the modules after the kernel
itself was already found. Calling failure at this point closes the Dwfl,
which isn't necessary. Just warn about the missing modules and continue
with the kernel only Dwfl.

Signed-off-by: Mark Wielaard <mark@klomp.org>