We didn't expect "error: no error", although this is
in fact a legitimate state (if something is erroneous
on the master, but not on the slave, e.g. INSERT fails
on master due to UNIQUE constraint which does not exist
on slave).

We now list the ignore for "0: no error" the same way
as any other ignore; moreover, if no or an empty
--slave-skip-errors is passed at start-up, we show
"OFF" instead of empty list, as intended. (The code
for that was there, but was only run for the empty-
argument case, even if it subsequently tested for
both conditions.) 

mysql-test/r/not_embedded_server.result:
  Show that passing no --slave-skip-errors results
  in "OFF" being shown as the variable's value. This
  test's "twin" (also not embedded, but setting
  --slave-skip-errors in its -opt file) lives in
  variables-notembbeded.test.
mysql-test/r/variables-notembedded.result:
  Show that error-ignore 0 is handled just like any other
  ignore. This test's "twin" (also not embedded, but not
  setting --slave-skip-errors in its -opt file) lives in
  not_embbeded_server.test.
mysql-test/t/not_embedded_server.test:
  Show that passing no --slave-skip-errors results
  in "OFF" being shown as the variable's value.
mysql-test/t/variables-notembedded-master.opt:
  Show that error-ignore 0 is handled just like any other
  ignore.
sql/slave.cc:
  - set up error-ignore-info even if --slave-skip-errors
    was not passed at start-up.
  
  - handle error 0 just like any other error.