feat(curl.sh): report errors, remove insecure flag

This changes the plugin a bit so that:
- it reports curl error log lines if the request fails
- curl doesn't use insecure mode (allows you to get alerts for invalid/expired certs and stops freaking me out)
- doesn't emit metrics in case of errors

Sample output:
```
$ bash curl_check.sh https://monitoring.api.rackspacecloud.com
status ok connection made
metric code string 200
metric time_connect double 0.145
metric time_total double 1.053
metric url string https://monitoring.api.rackspacecloud.com/
```
^ working case
```
$ bash curl_check.sh https://monitoring.api.rackspacecloud.com.nonexistent
status curl: (6) Could not resolve host: monitoring.api.rackspacecloud.com.nonexistent
```
^ error case. you'll get the error message in notifications.