fix(#87): allow getMessagesDescriptionsAndCodes for single Error

[annuh]

Fixes #87.

This PR fixes two issues:

1. Calling CifException::getMessagesDescriptionsAndCodes with a single Error:

Typed property Firstred\PostNL\Exception\CifException::$messages must not be accessed before initialization

2. Fix incorrect PostNL error response for single Error (and allow future changes...)

Received	According to PostNL API 😩

[codecov]

Codecov Report

Merging #88 (04d8ec5) into v2.0.x (1a1dc6c) will increase coverage by 0.06%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##             v2.0.x      #88      +/-   ##
============================================
+ Coverage     69.06%   69.12%   +0.06%     
  Complexity     2211     2211              
============================================
  Files           124      124              
  Lines          6429     6436       +7     
============================================
+ Hits           4440     4449       +9     
+ Misses         1989     1987       -2     

Files	Coverage Δ
src/Exception/CifException.php	100.00% <100.00%> (+20.00%)	⬆️
...seProcessor/Rest/AbstractRestResponseProcessor.php	56.92% <100.00%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1a1dc6c...04d8ec5. Read the comment docs.

[firstred]

Thanks a lot for the fix!