fix: throw better error for improper api reference

[keetonian]

Issue #, if available:
#1038

Description of changes:
Fixes an uncaught error if the RestApiId property contains a dictionary (like an import value statement). SAM does not yet support referencing an Api outside of the current template.

Description of how you validated changes:
Tests written, updated, and pass
Checklist:

• Write/update tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected
• Add/update example to examples/2016-10-31

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

[codecov-io]

Codecov Report

Merging #1224 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #1224      +/-   ##
===========================================
+ Coverage    94.43%   94.43%   +<.01%     
===========================================
  Files           72       72              
  Lines         3736     3738       +2     
  Branches       739      740       +1     
===========================================
+ Hits          3528     3530       +2     
  Misses         105      105              
  Partials       103      103

Impacted Files	Coverage Δ
samtranslator/plugins/api/implicit_api_plugin.py	98.61% <100%> (+0.01%)	⬆️

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