Improve lopsub error diagnostics.

Improve lopsub  error diagnostics.

If the lopsub package is not installed, the check for the lopsubgen
executable is the first thing that fails, so we print "lopsubgen
required" and exit without also showing the detailed information
which tells the user how to obtain the package.

Move the check down, so that the build fails with the detailed message,
and also educate the user that lopsub is also available as a Debian
package.