set.go: opportunity for panic in writeCrons when there's an error form GetSite

[Saxon Milton]

If for some reason the call to GetSite fails and the returned site is nil, then the filling of the data structs Timezone field will cause a panic where the site.Timezone field is accessed. We should immediately write the template with an unset data struct and error messaged when either GetSite or GetCronsBySite fails.

[Saxon Milton]

Resolved by PR #187