ENG-5956 sqlcmd error handling improvements

Restructured try/catch blocks and a few throws especially in the area of exec parameter validation, mainly to allow more consistent options for stopping immediately on error or soldiering on, and returning a proper exit code on actual errors. But also a little bit just to eliminate redundant code.

Did NOT address the clumsy code sharing between interactive and batch modes (source of batch mode bugs and lots of unwanted complexity) 
or the lack of code sharing between batch mode and the almost-yet-should-be-more identical "script file read" mode.

Did not address needless complexity and compute costs in the LIST commands, in procedure identification, and in literal string handling.
I'm pretty sure that quitting on errors as the default would just be seen as annoying when running interactively, so would you have different defaults in different modes? I think I suggested that to Andrew and he suggested instead we should keep it simple and have the one default behavior. I think I should probably poll a larger internal audience about this.