man: split the description of sd_bus_error_set()

man: split the description of sd_bus_error_set()

It was one giant all of text in pseudo-random order. Let's split it into
paragraphs talk about one subject each.

And unfortunately, the description of what happens when the error is not
set was not correct. In general, various functions treat 0/NULL as
not-an-error, and return 0.