Fix invalid filenames not printing an error

[ulmer-a]

This will resolve #232

[codecov]

Codecov Report

Merging #235 (0bac55e) into master (7e18300) will increase coverage by 0.06%.
The diff coverage is 98.87%.

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
+ Coverage   93.52%   93.58%   +0.06%     
==========================================
  Files          34       34              
  Lines        9054     9173     +119     
==========================================
+ Hits         8468     8585     +117     
- Misses        586      588       +2     

Impacted Files	Coverage Δ
src/index.rs	94.40% <ø> (ø)
src/index/visitor.rs	96.18% <0.00%> (-0.37%)	⬇️
src/lexer.rs	95.32% <ø> (ø)
src/parser.rs	97.86% <99.42%> (+0.27%)	⬆️
src/ast.rs	87.83% <100.00%> (+0.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a1c7ea6...0bac55e. Read the comment docs.