Error handling: Use &error_abort instead of NULL for errp parameters for may-not-fail invocations

Many functions accept an 'errp' parameter. Using "NULL" for the errp parameter in case "it cannot fail" should be avoided. If there is an unexpected error generated, it will simply be ignored. Making use of "&error_abort" instead allows us to catch unexpected errors.

The complete job is anything but bite-sized, but taking just a bite (.c file, or maybe subsystem) is easy enough.