rARC788098096e11

Fix PHP 8.0 ValueError calling mb_convert_encoding() with an invalid encoding

Summary:
Per https://www.php.net/manual/en/function.mb-convert-encoding.php, as of PHP 8.0.0, a ValueError is thrown if the value of `to_encoding` or `from_encoding` is an invalid encoding but a ValueError is not suppressed by the stfu operator ("@").

Origin of the function:

https://secure.phabricator.com/rPHU72ad8fd0f05b0d84f7d8efd7db62ad0b3ba4431f

Premising that Arcanist elevates warnings to exception, now we just try-catch.