Add information to failed api requests

Add addition information when requests for tag suggestions fail.

src/lua/endpoints/api_get.lua

@@ -36,7 +36,7 @@ local function api_get(req)
 		we're searching for, potentially causing a DoS with a
 		sufficiently backtrack-ey search/tag combination.
 		]]
-		assert(data:match("^[a-zA-Z0-9,%s-]+$"),"Bad characters in tag")
+		assert(data:match("^[a-zA-Z0-9,%s-]+$"),string.format("Bad characters in tag: %q",data))
 		return suggest_tags(req,data)
 	end
 end