Create and catch errors

[ronaldtse]

I think we need to elaborate these errors in interscript gem and catch them in the lambda.

• no system provided, no content provided
• conversion system specified doesn’t exist
• conversion process failure
• time-out (eg conversion too long)
• conversion text too long (need to define a max length)
• too many calls

Originally posted by @ronaldtse in https://github.com/interscript/infrastructure/issues/1#issuecomment-657009964