Raises an error if styleguide section comment misses section definition

[tow8ie]

Prior to this patch, if one tried to access a section (name) of a parsed KSS section comment that misses the section name definition, a NoMethodError: undefined method []' for nil:NilClass` was thrown because the code assumed the Regex for finding the section name always succeeded.

To handle this error of a missing or wrongly formatted section name catching the very generic NoMethodError is not specific enough. I therefore introduced a SectionNotDefinedError that can be catched instead.