Fix #53 - include empty header file

[TBladen]

Fix #53 by moving exception handling to the processing function, so that erroneous conditions (e.g. file not found) and an empty file (empty string) are handled differently. Includes an updated feature test case for regression testing.

[codecov-io]

Codecov Report

Merging #54 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master    #54      +/-   ##
========================================
- Coverage      98%    98%   -0.01%     
========================================
  Files          26     26              
  Lines        3202   3200       -2     
========================================
- Hits         3138   3136       -2     
  Misses         64     64

Impacted Files	Coverage Δ
shivyc/preproc.py	100% <100%> (ø)	⬆️

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[ShivamSarodia]

Thank you for this PR! Improving the ShivyC preprocessor is a high-priority task - see #47 for more.

Feel free to amend this PR to add yourself to the Contributors list in the README (you might need to rebase to see it). Overall LGTM.

[TBladen]

I've added myself and squashed the commits to make the pull request a bit cleaner. I'm definitely interested in doing more work on your project though, I was going to take a look at #37 later today :)

[ShivamSarodia]

Perfect, thanks :) I'll add some comments to #37 to describe a bit more about the preprocessor situation.