linux-initrd: Use 'call-with-error-handling' when booting.

* guix/build/linux-initrd.scm (canonicalize-device-spec): When label
  resolution fails, call 'error' instead of 'format' + 'start-repl'.
  (boot-system): Wrap most of body in 'call-with-error-handling'.
  Remove 'catch' around 'primitive-load' call.