exceptions: better handle unsupported macOS versions.

Don’t fail on `OS::ISSUES_URL` being missing (which happens on
unsupported macOS versions) and instead tell people to not file issues.

Library/Homebrew/exceptions.rb

@@ -356,7 +356,7 @@ class BuildError < RuntimeError
       return
     end
 
-    if formula.tap
+    if formula.tap && defined?(OS::ISSUES_URL)
       if formula.tap.official?
         puts Formatter.error(Formatter.url(OS::ISSUES_URL), label: "READ THIS")
       elsif issues_url = formula.tap.issues_url