Set ErrorHandling to ContinueOnError when gathering metrics from scripts

[Meroje]

When gathering metrics on a busy pool (or crashed/stopped) with script-collectors the fcgi error results in a 500 from the exporter, with the message as body.

An error has occurred during metrics collection:

[from Gatherer #2] dial unix /run/php-sock/php7.2-fpm.sock: connect: no such file or directory

By setting HandlerOpts we can have the error logged to stdout and metrics served as best-effort, in our case only php_fpm_up set to 0.

# HELP php_fpm_up Whether scraping PHP-FPM's metrics was successful.
# TYPE php_fpm_up gauge
php_fpm_up{socket_path="/run/php-sock/php7.2-fpm.sock"} 0

[Lusitaniae]

Thanks for the contribution @Meroje