Make verifynode() return the verification result through a separate
pointer, thus making it possible to signal errors using function
return value. Replace all fatal() and check_result() calls inside
verifynode() with zoneverify_log_error() calls and error handling code.
Add a REQUIRE assertion to emphasize verifynode() may be called with
some of its arguments set to NULL.
Modify all call sites of verifynode() so that its errors are properly
handled.
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