Check validity of both strings in two-string query

Currently Breakpoint only evaluates the first string in a two-string query. If you pass a two-string query to breakpoint() and neither string is valid, it assumes the 2nd is a valid query name. 

Added an `@else if` to _double-string-scss to check `$second` as well and return a warning if neither string passes `breakpoint-string-value()`.