[BUGFIX] Fix else return for nested issue reporting

Reference: #1380 

Updates the no-else-return rule to display multiple errors in cases of nested violations of this rule, where before it was only showing the most interior violation and would not display the others until that one was fixed.

@nzakas Please review, thanks! I'm not convinced this is the best implementation, but it seems to satisfy all the use-cases I could think of.

Fixes #1369
I think this is unnecessary. Here, you are traversing to find an if statement, but that already happens naturally by virtue of how the rule works. Nesting shouldn't matter because every if statement will be visited eventually. 

In general, rules should never traverse down into the AST, since ESLint does that for you.