Fixes exception bubbling for php7

[chrishulton]

I noticed some builds printing fatal errors, but exiting with status 0 and "Found 0 issues."

[DEBUG] phpmd:stable engine stderr: PHP Fatal error:  Uncaught TypeError: Argument 1 passed to PHPMD\PHPMD::setFileExtensions() must be of the type array, null given, called in /usr/src/app/Runner.php on
line 101 and defined in /usr/src/app/vendor/phpmd/phpmd/src/main/php/PHPMD/PHPMD.php:136
Stack trace:
#0 /usr/src/app/Runner.php(101): PHPMD\PHPMD->setFileExtensions(NULL)
...
  thrown in /usr/src/app/vendor/phpmd/phpmd/src/main/php/PHPMD/PHPMD.php on line 136

Analysis complete! Found 0 issues.

It looks like this is because the exception flow changed in php7:
https://trowski.com/2015/06/24/throwable-exceptions-and-errors-in-php7/
http://php.net/manual/en/language.errors.php7.php

The main point being there is now a separation between Exception and Error objects:

Throwable may be used in try/catch blocks to catch both Exception and Error objects (or any possible future exception types).

interface Throwable
    |- Exception implements Throwable
        |- ...
    |- Error implements Throwable
        |- TypeError extends Error
        ...

Rescuing the globally namespaced \Throwable will also rescue Errors such as this TypeError. I can verify this leads to the correct behavior (the error message is also printed in our format from the catch block):

Running phpmd: Done!
error: (CC::Analyzer::Engine::EngineFailure) engine phpmd:stable failed with status 1 and stderr
Exception: Argument 1 passed to PHPMD\PHPMD::setFileExtensions() must be of the type array, null given, called in /usr/src/app/Runner.php on line 101 in /usr/src/app/vendor/phpmd/phpmd/src/main/php/PHPMD/PHPMD.php
#0 /usr/src/app/Runner.php(101): PHPMD\PHPMD->setFileExtensions(NULL)
...

I will make the same fix in phpcodesniffer assuming this fix goes well.