nmbug-status: Avoid hard-coded filename in error message

nmbug-status: Avoid hard-coded filename in error message

We already have a 'filename' variable with the name, so stay DRY and
use that variable here.

Also fix a missing-whitespace error from bed8b674 (nmbug-status:
Clarify errors for illegible configs, 2014-05-10), wrapping on the
sentence to match similar error-generation earlier in this function.