fix(import): Show details for import error

[abrain]

If wp_insert_post returns additional details in the WP_Error object, show them to the user. Also let the user know how many imports were successful before the error.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 25.67%. Comparing base (9ca06ab) to head (0140a1e).
Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/Import/Helper.php	0.00%	5 Missing ⚠️
src/Import/Tool.php	0.00%	5 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #338      +/-   ##
=============================================
+ Coverage      25.64%   25.67%   +0.02%     
- Complexity      1607     1610       +3     
=============================================
  Files             81       82       +1     
  Lines           6223     6236      +13     
=============================================
+ Hits            1596     1601       +5     
- Misses          4627     4635       +8     

Flag	Coverage Δ
integration	11.31% <0.00%> (-0.03%)	⬇️
unit	15.60% <33.33%> (+0.04%)	⬆️

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