Fail with config ->errstr() when undef read

[tejr]

The Config::Tiny parent class for Monitoring::Plugin::Config does not necessarily raise an exception in $@/$EVAL_ERROR on a failed call to its ->read() method. Indeed, it does not in most cases, at least in the most recent Config::Tiny.

If a file does not exist or for whatever other reason cannot actually be read--such as permissions problems--Monitoring::Plugin ends up reporting a "Missing config section" to the caller, which is misleading.

This information is available from the ->errstr() method of the base class, however, and an inspection of the code for the ->read() method in the parent class suggests the correct approach is to look for a return of undef from the ->read() method, and use the return value of ->errstr() as the reported error for ->_die(). This commit adds such a check.

To reproduce, given the following plugin mptest:

use strict;
use warnings;
use Monitoring::Plugin qw(%ERRORS);
my $mp = Monitoring::Plugin->new(
    usage => '',
);
$mp->getopts;
$mp->plugin_exit($ERRORS{OK}, 'Done!');

Running it without options of course works, like so:

$ perl mptest
MPTEST OK - Done!

However, prior to this patch, if we specify --extra-opts with a nonexistent filename, we get a misleading error:

$ perl mptest --extra-opts=@/nonexistent
Invalid section 'mptest' in config file '/nonexistent'

With this patch included, the error is more accurate and helpful:

$ perl mptest --extra-opts=@/nonexistent
Failed to open file '/nonexistent' for reading: No such file or directory

[sni]

good catch, thank you.