check_prob() for an argument that is a vector rather than one number, such
as the credibility levels of expected_coverage(). A level outside (0, 1) produces one meaningless number in a table of plausible ones:
levels = c(-1, 2) scores the empty interval and the whole line, and comes
back as coverage 0 and 1 with nothing said. The message lists the values, as
check_counts() does, since which entry is wrong is the thing worth reading.