fix: correct QnAMaker response types and expose error from server

[chon219]

Fixes #2317

Description

This PR exposed error message from QnAMaker server to allow users locating and fixing errors.
Response type of HttpRequestUtils of QnAMaker is not correctly defined, it should be QnAMakerResults (while calling generate api) or undefined (while calling train api). Check out https://docs.microsoft.com/en-us/rest/api/cognitiveservices/qnamaker4.0/runtime/generateanswer#response . This PR will fix this.

Specific Changes

• Exposed error from QnAMaker server side
• Corrected return type to QnAMakerResults

Testing

[coveralls]

Pull Request Test Coverage Report for Build 546784800

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.002%) to 84.872%

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Files with Coverage Reduction	New Missed Lines	%
libraries/botbuilder-ai/src/qnamaker-utils/generateAnswerUtils.ts	1	86.49%
libraries/adaptive-expressions/src/triggerTrees/trigger.ts	2	61.92%

Totals
Change from base Build 546673406:	-0.002%
Covered Lines:	18282
Relevant Lines:	20559

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💛 - Coveralls

[joshgummersall]

Two quick suggestions.