Previously, when reading an incompatible "parameters" file, 'guix
system' would print a warning and then crash with a wrong-type-arg
backtrace because code expects 'read-boot-parameters' to always return a
<boot-parameters> record.
* gnu/system.scm (read-boot-parameters): Upon incompatibility, raise an
error instead of returning #f. Also raise a '&fix-hint' condition.
* tests/boot-parameters.scm ("read, construction, mandatory fields"):
Define 'test-read-boot-parameters' as a macro; expect
'formatted-message?' exceptions rather than #f returns.
