Full TCL return/error protocol

[wduquette]

Standard TCL's return and error methods allow TCL code to return and process any valid return code (including break and continue) which is important when implementing control structures, as well as error codes on error returns. It isn't clear that Molt needs all of this machinery, but ultimately we probably want to provide it.

[wduquette]

Note: Molt does provide equivalents of all of the standard TCL return codes. It's the ability to define arbitrary return codes that's lacking.