Add an --on-error and \onerror to control how the CLI returns errors to
the user. Including tests.
Address merge comments and add a few updates.

- Make the \i command return the exit code so a failure in the included
file get propgated out to the caller

- Add an optional exit code parameter to \q to allow exiting with a
fixed return code. 

- make \onerror with no parameters print current status,
- make \onerror CONTINUE with no status work correctly. 
- add more error details for bad version of \onerror

And, of course, update the tests with new functionality.
merge tjoneslo: Add an --on-error and \onerror to control how the CLI returns errors

Merge pull request #55 from FoundationDB/sql-layer-client-tools/on-error-backslash
https://github.com/FoundationDB/sql-layer-client-tools/pull/55