Catch premature dialog endings

[mdrichardson]

Fixes #3355

Description

runDialog() works great for dialogs that take more than one turn to complete, but it fails otherwise. A single-step or immediately-cancelled dialog should be pretty rare, but this should fix things for them.

The general flow for the way things work currently:

1. bot.js file calls runDialog() (usually in onTurn() or onMessage()
2. runDialog sets result to continueDialog()
3. If no dialogs have started (normal), the result will be DialogTurnStatus.Empty, so runDialog then calls beginDialog().
4. Normally, the turn completes and when the user messages the bot to respond to a dialog prompt, runDialog() is called again, so continueDialog() is called again.
   • However, if--after first calling beginDialog()--the dialog is cancelled or ended before the turn ends, result remains DialogTurnStatus.Empty (instead of getting updated to Completed or Cancelled), so EoC is never sent back to the parent

Specific Changes

Allow result to be updated with the beginDialog() call.

Testing

Tested using the bots in the linked issue and it worked.

[coveralls]

Pull Request Test Coverage Report for Build 625821802

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.003%) to 84.887%

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Files with Coverage Reduction	New Missed Lines	%
libraries/botbuilder-lg/src/templateExtensions.ts	1	84.48%

Totals
Change from base Build 625820512:	-0.003%
Covered Lines:	18646
Relevant Lines:	20923

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💛 - Coveralls

[stevengum]

Small changes and cleanup requested, otherwise it looks great! Thanks for the quick turnaround 👍🏼

[stevengum]

🚀