Use error_is_set() only when necessary (again)

Use error_is_set() only when necessary (again)

error_is_set(&var) is the same as var != NULL, but it takes
whole-program analysis to figure that out.  Unnecessarily hard for
optimizers, static checkers, and human readers.  Commit 84d18f0 dumbed
it down to obvious, but a few more have crept in since, and
documentation was overlooked.  Dumb these down, too.

Signed-off-by: Markus Armbruster <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>