ktest: Add warning when bugs are ignored

When IGNORE_ERRORS is set, ktest will not fail a test if a backtrace
is detected. But this can be an issue if the user added it in the
config but forgot to remove it. They may be left wondering why their
test did not fail, or even worse, why their bisect gave the wrong
commit.

Add a warning in the output if IGNORE_WARNINGS is set, and ktest detects
a kernel error.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>