Improve the formatting of error messages when rules fail to load

Slide: https://docs.google.com/presentation/d/1ijAWBKBJi90j6eCjHdkk-y22gGjzJZ2J7XVQcZejaiI/edit#slide=id.g1d8fe0edb4_0_38

Suggested text:

Invalid rule file(s) found:

• /opt/lala/file_one.windup.xml
• /opt/lala/file_two.windup.xml

Also, make sure that these are not passed to windup core for analysis.