Return an error when loadAppendOnlyFiles fails instead of
exiting. DEBUF LOADAOF command is only meant to be used by
the test suite, and only by tests that generated an AOF file
first. So this change is ok (considering that the caller is
likely to catch this error and die).
This actually revert part of the code in #9012, and now
DEBUG LOADAOF behaves the same as DEBUG RELOAD (returns an
error when the load fails).
Plus remove a `after 2000` in a test, which can save times (looks like copy paste error).
66bed3f220