grub-install: focus error messages on what the user did wrong

[Ben Armstrong]

The user would be greatly aided in finding the real problem when
grub-install fails if more care were taken to determine what the user
actually did wrong and issue an appropriate error message.

In particular, one user asking for help on irc on channel #hurd
(irc.openprojects.net) complained that when he specified:

grub-install --root-directory=/boot '(hd0)'

He got this error message:

Probing devices to guess BIOS drives. This may take a long time.
/dev/hda1 does not have any corresponding BIOS drive.

But on close examination of the grub-install script, I discovered that
it greps install.map.  The non-sensical --root-directory he specified
was causing grub-install to look in the wrong directory for install.map.

There should therefore be a file existence check on device.map and
an appropriate error message issued if it is missing, e.g.

Cannot find /boot/boot/grub/device.map.  Wrong --root-directory specified?

Thanks,
Ben Armstrong
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