Magento 2 - How to view static content deploy errors

I reckon there's several types of errors that can be triggered by the deployment.

First you can see that Exception are handled directly in the execution in Magento/Deploy/Console/Command/DeployStaticContentCommand.php:

    catch (\Exception $e) {
        $output->writeln('<error>' . $e->getMessage() . '</error>>');
        if ($output->getVerbosity() >= OutputInterface::VERBOSITY_VERBOSE) {
            $output->writeln($e->getTraceAsString());
        }
        return;
    }

Regarding the errors that are counted it's a totally different system, you need to look into the Magento/Deploy/Model/Deployer class, the output you got is written by the deploy() method:

$this->output->writeln("\nSuccessful: {$this->count} files; errors: {$this->errorCount}\n---\n");

Now if we check when this errorCount variable is incremented, we find the following in the deployFile() method:

    catch (\Exception $exception) {
        $this->output->write('.');
        $this->verboseLog($exception->getTraceAsString());
        $this->errorCount++;
    }

And as you may have already guessed, the verboseLog() method only outputs when the command is run on verbose mode:

private function verboseLog($message)
{
    if ($this->output->isVerbose()) {
        $this->output->writeln($message);
    }
}

So I reckon you need to run your command like this to see the errors:

php bin/magento setup:static-content:deploy -v

If it doesn't work try the extra verboses:

php bin/magento setup:static-content:deploy -vv
php bin/magento setup:static-content:deploy -vvv

You can often see errors within your less by viewing the css file that is generated on your development environment. This doesn't quite answer the question but will hopefully be helpful to someone.