fstab-generator: make error more helpful in case of duplicates in fstab

fstab-generator: make error more helpful in case of duplicates in fstab

Traditional sysvinit systems would not complain about duplicates in
fstab. Rather it (through monut -a) would mount one fs on top of another,
in effect the last entry taking precedent.

In systemd, the first entry takes precedent, all subsequent ones are
ignored and an error is printed.

The change of behavior and the source of this error message was causing
some confusion, so give a hint what migt be wrong.