Add stack trace for previous exceptions to bootstrap error message

[Steveb-p]

Addendum to #4620 and #4878.
Closes #5345

Adds previous exception stacktraces into the output, until there is no more left.

 Error in bootstrap script: PHPUnit\TestFixture\MyException:
 Big boom. Big bada boom.
 #0 /home/pniedzielski/PhpstormProjects/phpunit/src/Util/FileLoader.php(66): include_once()
 #1 /home/pniedzielski/PhpstormProjects/phpunit/src/Util/FileLoader.php(49): PHPUnit\Util\FileLoader::load()
 #2 /home/pniedzielski/PhpstormProjects/phpunit/src/TextUI/Command.php(565): PHPUnit\Util\FileLoader::checkAndLoad()
 #3 /home/pniedzielski/PhpstormProjects/phpunit/src/TextUI/Command.php(402): PHPUnit\TextUI\Command->handleBootstrap()
 #4 /home/pniedzielski/PhpstormProjects/phpunit/src/TextUI/Command.php(112): PHPUnit\TextUI\Command->handleArguments()
 #5 /home/pniedzielski/PhpstormProjects/phpunit/src/TextUI/Command.php(97): PHPUnit\TextUI\Command->run()
 #6 Standard input code(11): PHPUnit\TextUI\Command::main()
 #7 {main}
 
 Previous exception: Exception:
 Previous boom.
 #0 /home/pniedzielski/PhpstormProjects/phpunit/src/Util/FileLoader.php(66): include_once()
 #1 /home/pniedzielski/PhpstormProjects/phpunit/src/Util/FileLoader.php(49): PHPUnit\Util\FileLoader::load()
 #2 /home/pniedzielski/PhpstormProjects/phpunit/src/TextUI/Command.php(565): PHPUnit\Util\FileLoader::checkAndLoad()
 #3 /home/pniedzielski/PhpstormProjects/phpunit/src/TextUI/Command.php(402): PHPUnit\TextUI\Command->handleBootstrap()
 #4 /home/pniedzielski/PhpstormProjects/phpunit/src/TextUI/Command.php(112): PHPUnit\TextUI\Command->handleArguments()
 #5 /home/pniedzielski/PhpstormProjects/phpunit/src/TextUI/Command.php(97): PHPUnit\TextUI\Command->run()
 #6 Standard input code(11): PHPUnit\TextUI\Command::main()
 #7 {main}

[codecov]

Codecov Report

Merging #5346 (29827ea) into 9.6 (c993f0d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                9.6    #5346      +/-   ##
============================================
+ Coverage     83.18%   83.20%   +0.01%     
- Complexity     4616     4617       +1     
============================================
  Files           271      271              
  Lines         13662    13674      +12     
============================================
+ Hits          11365    11377      +12     
  Misses         2297     2297              

Impacted Files	Coverage Δ
src/TextUI/Command.php	74.89% <100.00%> (+0.64%)	⬆️

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