Handle errors and warnings before preflight check

Sometimes output of nagios -v nagios.cfg returns errors before the
preflight check, like this for example:

...
Reading configuration data...
Read main config file okay...
Error: Cannot open config file '/etc/nagios/services/service-XXX.cfg'
for reading: Permission denied
Error processing object config files!

***> One or more problems was encountered while processing the config
files...
...

These changes handle this situation.

Additionally, added the option to add the output of the nagios -v
command to the output of the check, excluding all the lines starting
with 'Processing'.
Merge branch 'improvements' of https://github.com/abooitt/check_nagios_config into improvements