* guix/scripts/system.scm (read-operating-system) <catch handler>: Add
case for 'syntax-error'. Correct message for default case.
* guix/scripts/offload.scm (build-machines) <catch handler>: Add case
for 'syntax-error'.
* tests/guix-system.sh: New file.
* Makefile.am (SH_TESTS): Add it.