Catch CodeRay exception for invalid language

CodeRay throws an exception if you give it a language it doesn't find a plugin
for. Reported as rubychan/coderay#245

Fixes openSUSE#6960

Co-authored-by: Dany Marcoux <dmarcoux@suse.com>

src/api/lib/obsapi/markdown_renderer.rb

@@ -41,7 +41,9 @@ def link(link, title, content)
 
     def block_code(code, language)
       language ||= :plaintext
-      CodeRay.scan(code, language).div(css: :class)
+      CodeRay.scan(code, language).div
+    rescue ArgumentError
+      CodeRay.scan("```#{language}#{code}", :plaintext).div unless language == :plaintext
     end
   end
 end