Fail with config ->errstr() when undef read

Fail with config ->errstr() when undef read

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