Validate that Phar exists upstream before executing update

[grasmash]

We recently had our phar build job on Travis fail. The release was made, but the phar was not attached. Running the self-update command produced:

Notice: Undefined offset: 0 in phar:///usr/local/bin/acli/vendor/consolidation/self-update/src/SelfUpdateCommand.php on line 94

Notice: Trying to get property 'browser_download_url' of non-object in phar:///usr/local/bin/acli/vendor/consolidation/self-update/src/SelfUpdateCommand.php on line 94
Downloading Acquia CLI (acquia/cli) 1.7.1

Fatal error: Uncaught TypeError: Argument 1 passed to Symfony\Component\Filesystem\Filesystem::copy() must be of the type string, null given, called in phar:///usr/local/bin/acli/vendor/consolidation/self-update/src/SelfUpdateCommand.php on line 150 and defined in phar:///usr/local/bin/acli/vendor/symfony/filesystem/Filesystem.php:37
Stack trace:
#0 phar:///usr/local/bin/acli/vendor/consolidation/self-update/src/SelfUpdateCommand.php(150): Symfony\Component\Filesystem\Filesystem->copy(NULL, '/usr/local/bin/...')
#1 phar:///usr/local/bin/acli/vendor/symfony/console/Command/Command.php(255): SelfUpdate\SelfUpdateCommand->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#2 phar:///usr/local/bin/acli/vendor/symfony/console/Application.php(989): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#3 phar:///usr/local/bin/acli/vendor/symfony/console/Application.p in phar:///usr/local/bin/acli/vendor/symfony/filesystem/Filesystem.php on line 37

Note the Symfony\Component\Filesystem\Filesystem->copy(NULL, '/usr/local/bin/...')

The self-update library should fist check to see if the phar exists, and if it does not, it should throw an informative exception like "No Phar file is attached to the x.y.z release on GitHub."