Do not call exit() upon verifynode() errors

Do not call exit() upon verifynode() errors

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.