nymms/config/yaml_config tests file access poorly

This bit of code:

```
if os.path.isfile(f):
logger.debug("Parsing include (%s:%d): %s",
filename, lineno, f)
c.extend(recursive_preprocess(f))
else:
logger.warning("%s is not a regular file, "
"skipping (%s:%d).", f, filename,
lineno)
```

Will give the error if, for example, it cannot access the file due to not having permissions to the directory the file is in. Need a better set of checks and error messages. try/open/except maybe?