Added option to ignore folders without style definitions during theme compilation

Relates to #7848

This problem still persists in Vaadin 8.7.2 and applies to Subversion, CVS, Dimensions and probably others.
I added the ignoreNonThemeFolders option to the vaadin-maven-plugin and created PRs in vaadin/maven-plugin and vaadin/sass-compiler.
(https://github.com/vaadin/maven-plugin/pull/116 and https://github.com/vaadin/sass-compiler/pull/321)

With this option set to true the plugin looks for styles.scss in all folders under src/main/webapp/VAADIN/themes and will ignore failures due to folders created by versioning systems that do not contain a style definition. Information about failed attempts is printed to the console.

You can use it via

  <configuration>
        <ignoreNonThemeFolders>true</ignoreNonThemeFolders>
   </configuration>

in the vaadin-maven-plugin

Start with vaadin/maven-plugin#116 and vaadin/sass-compiler#321, since the proposed work is already located in those repositories rather than this issue's repository. Check the handling of ignoreNonThemeFolders and the documented Maven configuration; done means the option is available and failed attempts for non-theme folders are reported as described.