vmimage.subr: noisier failure for unsupported targets

The return code of write_partition_layout() doesn't bubble up, so an
invocation of make vm-release for an incorrect/unsupported target will
appear to succeed while make vm-install will fail due to missing
files. This isn't a common point of failure, but is worth handling
properly.

Upgrade this case to print a message to stderr, and exit in place. This
is okay to do since at this point in the execution of mk-vmimage.sh,
cleanup() has already been run.

Reviewed by: gjb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D26985