test-ethernet: rework error handling

Align a bit more the error handling in test-ethernet with the other tests:

- Turn on `-eu -o pipefail' globally.
- Remove the custom error handler.
- In all the cases where ethernet-parser.py returns an error code, handle
  it explicitly with an `if' construct.

This should be more readable and allow to catch more (unexpected) errors.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>