Using the 'environment' property on a cron resource breaks on Solaris

When using the environment property, Puppet adds the value of the property as a line above the cron job. This works properly in linux, but cron in Solaris doesn't support this, which produces an error containing something like

crontab: error on previous line; unexpected character found in line.

crontab: errors detected in input, no crontab file generated.

This isn't particularly useful. It seems like we could catch this earlier and raise a more useful error, something like "The `environment` property is not supported for `cron` resources on Solaris" rather than just bubbling up the less-clear error that bubbles up from the system.