sdjournal: Provide error messages as strings

[stevenjm]

This was previously just displaying the numeric errno value, resulting in error messages like:

failed to seek to cursor "foo\n": 22

The syscall.Errno type provides a handy function for returning the string value of an error, so just use that for more informative messages.

[lucab]

Thanks for the PR! Just for clarity, this wouldn't provide any more information about the error, right? It only does the equivalent of strerror(3) (errno int->string). Was that your goal?

[stevenjm]

Correct, and indeed it is my goal. By "more informative" I meant that the message is more informative at a glance, not that there's additional information supplied.

[lucab]

@stevenjm ack, LGTM!