Add missing exit() in pg_verifybackup when failing to find pg_waldump

Add missing exit() in pg_verifybackup when failing to find pg_waldump

pg_verifybackup needs by default pg_waldump to check after a range of
WAL segments required for a backup, except if --no-parse-wal is
specified.  The code checked for the presence of the binary pg_waldump
in an installation and reported an error, but it forgot to properly
exit().  This could lead to confusing errors reported.

Reviewed-by: Robert Haas, Fabien Coelho
Discussion: https://postgr.es/m/YQDMdB+B68yePFeT@paquier.xyz
Backpatch-through: 13