Avoid inline error checking (#5752)
Avoid inline error checking (#5752)

Update our contributing guidelines to state that we prefer to
do error checking in a separate stanza, rather than doing error
assignment and checking on the same line. At this time, there
are less than 400 instances of the latter (unpreferred) pattern
in the Boulder codebase, and 1300+ instances of the former
(preferred) pattern.
jcjones/boulder,jsha/boulder,letsencrypt/boulder,jsha/boulder,jcjones/boulder,letsencrypt/boulder,jsha/boulder,jcjones/boulder,jcjones/boulder,letsencrypt/boulder