Fix #154: pidstat: Don't stop if /proc/#/schedstat files not found

Fix #154: pidstat: Don't stop if /proc/#/schedstat files not found

Commit a41b24a added %wait field to pidstat CPU statistics. This field
is calculated for each task using the contents of /proc/#/schedstat
file. Yet pidstat should not stop if this file cannot be found: It
should display the other CPU statistics as usual and display 0.00 for
this particular field. This is what this patch does.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>