Throw BuidlerPluginError instead of Error

[fvictorio]

Instead of:

    if (process.exitCode > 0) throw new Error(ui.generate('tests-fail', [process.exitCode]));

consider doing

    if (process.exitCode > 0) throw new BuidlerPluginError('solidity-coverage', ui.generate('tests-fail', [process.exitCode]));

Relevant section in the docs: https://buidler.dev/advanced/building-plugins.html#throwing-errors-from-your-plugins

[cgewecke]

This is fixed for the Hardhat plugin in 0.7.12. Thanks for highlighting @fvictorio!