Throw an exception if code coverage fails to write to disk

[duncan3dc]

I'm experiencing the same issue as #692 (seems to the \SoapClient causing some issues).

I traced the error to this \DomDocument#save() call.

This PR adds error checking to that call, so that at least the issue is reported.

Before:

PHPUnit 8.3.5 by Sebastian Bergmann and contributors.
.                                                                   1 / 1 (100%)
Time: 46 ms, Memory: 6.00 MB
OK (1 test, 1 assertion)

Generating code coverage report in PHPUnit XML format ... done [8 ms]

After:

PHPUnit 8.3.5 by Sebastian Bergmann and contributors.
.                                                                   1 / 1 (100%)
Time: 44 ms, Memory: 6.00 MB
OK (1 test, 1 assertion)

Generating code coverage report in PHPUnit XML format ... Unable to save the XML
flush error

[codecov]

Codecov Report

Merging #700 into 8.0 will increase coverage by 1.55%.
The diff coverage is 60.00%.

@@             Coverage Diff              @@
##                8.0     #700      +/-   ##
============================================
+ Coverage     82.29%   83.84%   +1.55%     
- Complexity      832      856      +24     
============================================
  Files            37       39       +2     
  Lines          2496     2538      +42     
============================================
+ Hits           2054     2128      +74     
+ Misses          442      410      -32     

Impacted Files	Coverage Δ	Complexity Δ
src/Report/Xml/Facade.php	91.85% <60.00%> (-2.60%)	31.00 <0.00> (+2.00)	⬇️
src/CodeCoverage.php	68.42% <0.00%> (-3.75%)	161.00% <0.00%> (-20.00%)
src/Node/Builder.php	97.14% <0.00%> (-0.08%)	24.00% <0.00%> (ø%)
.../Exception/UnintentionallyCoveredCodeException.php	100.00% <0.00%> (ø)	4.00% <0.00%> (ø%)
src/Driver/PHPDBG.php
src/Driver/PCOV.php
src/Driver/Xdebug.php
src/ProcessedCodeCoverageData.php	100.00% <0.00%> (ø)	27.00% <0.00%> (?%)
src/RawCodeCoverageData.php	100.00% <0.00%> (ø)	9.00% <0.00%> (?%)
src/Driver/PcovDriver.php	55.55% <0.00%> (ø)	3.00% <0.00%> (?%)
... and 6 more

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[sebastianbergmann]

This pull request has conflicts that must be resolved. Please update your pull request and base it on the 8.0 branch. Thanks!

[sebastianbergmann]

Merged manually, thanks.