Malformed UTF-8 characters, possibly incorrectly encoded

[adrenalinkin]

When some source files contains not UTF-8 symbols we can't send coverage report

Collect git info
Dump submitting json file: /app/tests/_output/coveralls-upload.json
Fatal error:
Method PhpCoveralls\Bundle\CoverallsBundle\Entity\JsonFile::__toString() must not throw an exception,
caught UnexpectedValueException: Can not encode to JSON, error:
"Malformed UTF-8 characters, possibly incorrectly encoded".
If you have non-UTF8 chars, consider migration to UTF8.
in /app/vendor/php-coveralls/php-coveralls/src/Bundle/CoverallsBundle/Api/Jobs.php on line 117

But also we have no idea where is problem locates!
In my case we have over than 5 thousand files.

I propose implement more friendly message in that case when json_encode has been failed