Do not call exit() upon NSEC3 verification errors

Do not call exit() upon NSEC3 verification errors

Make verifynsec3(), verifynsec3s(), and verifyemptynodes() return the
verification result through a separate pointer, thus making it possible
to signal errors using function return values.  Replace all
check_result() and fprintf() calls inside these functions with
zoneverify_log_error() calls and error handling code.

Modify all call sites of verifynsec3(), verifynsec3s(), and
verifyemptynodes() so that their errors are properly handled.