sysroot: Add a clearer error if /boot/loader isn't found

sysroot: Add a clearer error if /boot/loader isn't found

I've seen people confused by this error in the case where
`/boot` isn't mounted or the BLS fragments were deleted, etc.
If you understand ostree deeply it's clear but, let's do
better here and a direct error message for the case where
we can't find `/boot/loader` which is the majority of these.

The other case could happen if e.g. just the BLS fragment
for the booted deployment was deleted; let's reword that
one a bit too.

Closes: #1905
Approved by: rfairley