create well-formed JSON with metadata whenever there's an error in the user's code

[pgbovine]

Properly reporting errors is very important since lots of times users will enter code with errors. Specifically:

• detect syntax errors and report a precise error message, along with the line / column number info about where the error occurred, so that frontend can report to user
• report runtime errors and exceptions with the proper error messages / backtraces / etc. to help the user debug
• [optional] if the user's code runs for too long or too many steps, report partial results and an error message (although the backend executor can also handle timeouts, albeit at a coarser-grained level)

[pgbovine]

sean fixed