Display more accurate AJAX alert message

Stuff missing:

• error message when Template Preview call fails (now error is on timeout only, not on 500 status)
• More descriptive error message - we do get the error message, let's display it instead of "Temporaty network problem" (error 500 is usually not temporary )
• error messages stack when Issue Type dropdown change causes AJAX problem (should be just 1 message i think, but they are closeable so maybe not)

Nothing urgent.