Add warning if npm is not in $PATH

When `npm` is not found in the PATH, the `eval` statement in the npm plugin silently fails, leaving the user puzzled because npm completion doesn't work even though the npm command works fine.

This also goes hand in hand with the fact that the default zshrc has an `export PATH` statement *after* having sourced oh-my-zsh.

With this pull request, the plugin will fail to init with an explicit error, so the user knows he has to add `npm` to the PATH.