initramfs-framework: better error reporting for invalid root boot parameter

initramfs-framework: better error reporting for invalid root boot parameter

When the "boot" parameter refers to a non-existent device, the only
visible output at normal log levels was a rather confusing:
   ERROR: There's no '/dev' on rootfs.

That's because the actual error, not being able to find the root
device, was only a debug message, which gets ignored in the default
mode.

Promoting the "root '$bootparam_root' doesn't exist." message from
"debug" to "msg" gives sufficient context to understand the error. A
more intrusive change would be to change also the control flow.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>