Report compilation errors and optionally fail build

Thank you for the sass-maven-plugin. I find it very useful and simple to use.

I missed the functionality to fail a maven build if the sass/scss compilation fails. This change implements reporting of compilation errors in the maven output and a new configuration option "failOnError", which allows failing the maven build if set to true. For backwards compatibility I set its default value to false.

I would be happy to see this change in the next release of the sass-maven-plugin!

Thanks,
Julian