Don't log "symbol 'x' is not defined by script 'y'" error when terminating

When a JS environment (e.g. server, jsexec) is terminated, it's possible
that a require() script was being evaluated. But since termination would
abort that evaluation, it's not unexpected if a symbol ends up not being
defined before the require() script was terminated, so don't report an
error in that case.

Fix issue #681