Improve error-handling of Mongo shell commands

[StephenWeatherford]

This especially should provide accurate error messages instead of "timed out" when the "use db" command fails.

Fixes #838
Also fixes #1104

Possibly fixes error reporting for:
#1071
#988
#852
#820
#794
#1092

The main part is a near-rewrite of shell.ts.

Testcases in OneNote:

[StephenWeatherford]

@EricJizbaMSFT @PrashanthCorp @nturinski FYI, I was trying to get this finished up yesterday and also greatly simply the algorithm by passing the script to be processed through the command line instead of stdin (which would allow me to use Eric's cpUtils unchanged. Unfortunately, it causes some differences in output that I don't think I can overcome. The problem with using cpUtils is that as currently written the mongo shell execution requires an interactive handshaking with stdin/stdout. Probably giving it a rest until next week, but want to get it out of the way. Also wondered if it would make sense to use the vscode terminal for this (but we'd have to be able to read the output).

[ejizba]

That would've been great if we could just re-use cpUtils, but honestly not that surprised that it didn't work. Oh well ¯\_(ツ)_/¯

[StephenWeatherford]

That would've been great if we could just re-use cpUtils, but honestly not that surprised that it didn't work. Oh well ¯_(ツ)_/¯

It was so close.

[StephenWeatherford]

Superseded by #1148