Raise InvalidEventException when RestApiId/ApiId is not resolved to a string

[aahung]

… string

Issue #, if available:

Description of changes:

Description of how you validated changes:

Checklist:

• Add/update tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.42%. Comparing base (e7a1496) to head (1b389dd).
Report is 1188 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2213      +/-   ##
===========================================
+ Coverage    93.58%   94.42%   +0.83%     
===========================================
  Files           90       95       +5     
  Lines         6124     6561     +437     
  Branches      1260     1325      +65     
===========================================
+ Hits          5731     6195     +464     
+ Misses         183      170      -13     
+ Partials       210      196      -14     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.