feat: Added clearer errors for incorrect image reading

[fg-mindee]

This PR introduces the following modifications:

• added specific image reading errors: as pointed out by @mohamedmindee, cv2.imread yields a None whether the file is incorrectly located or of the wrong type, which is now fixed
• added specific unittests to check the new behaviour

Any feedback is welcome!

[codecov]

Codecov Report

Merging #79 (f058815) into main (14279aa) will increase coverage by 0.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   93.90%   94.44%   +0.54%     
==========================================
  Files          24       24              
  Lines         787      792       +5     
==========================================
+ Hits          739      748       +9     
+ Misses         48       44       -4     

Flag	Coverage Δ
unittests	94.44% <100.00%> (+0.54%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
doctr/documents/reader.py	100.00% <100.00%> (ø)
doctr/models/core.py	85.71% <0.00%> (+8.16%)	⬆️

---

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 14279aa...e210f5a. Read the comment docs.