This commit improves pgbench \sleep command so that it handles
the following three cases more properly.

(1) When only one argument was specified in \sleep command and
    it's not a number, previously pgbench reported a confusing error
    message like "unrecognized time unit, must be us, ms or s".
    This commit fixes this so that more proper error message like
    "invalid sleep time, must be an integer" is reported.

(2) When two arguments were specified in \sleep command and
    the first argument was not a number, previously pgbench treated
    that argument as the sleep time 0. No error was reported in this
    case. This commit fixes this so that an error is thrown in this
    case.

(3) When a variable was specified as the first argument in \sleep
    command and the variable stored non-digit value, previously
    pgbench treated that argument as the sleep time 0. No error
    was reported in this case. This commit fixes this so that
    an error is thrown in this case.

Author: Kota Miyake
Reviewed-by: Hayato Kuroda, Alvaro Herrera, Fujii Masao
Discussion: https://postgr.es/m/23b254daf20cec4332a2d9168505dbc9@oss.nttdata.com