[Fix] `nvm-exec`: properly exit when failing; improve error messages, suppress `nvm use --help` output
[Fix] `nvm-exec`: properly exit when failing; improve error messages, suppress `nvm use --help` output

Fixes #997.
PeterDaveHelloKitchen/nvm,edwmurph/nvm,dfackler/nvm,lukechilds/nvm,PeterDaveHelloKitchen/nvm,dfackler/nvm,PeterDaveHello/nvm,lukechilds/nvm,creationix/nvm,Theodeus/nvm,Theodeus/nvm,PeterDaveHello/nvm,creationix/nvm,ljharb/nvm,ljharb/nvm,evocateur/nvm,tlevine/nvm,evocateur/nvm,edwmurph/nvm,tlevine/nvm