fix: add periods to error messages, fix quotes

[keetonian]

Issue #, if available:
N/A

Description of changes:
Update error messages

Description of how you validated changes:
Made error messages more grammatically correct and easier to read.

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 #1430 into develop will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff            @@
##           develop    #1430   +/-   ##
========================================
  Coverage    94.37%   94.37%           
========================================
  Files           78       78           
  Lines         4678     4678           
  Branches       934      934           
========================================
  Hits          4415     4415           
  Misses         121      121           
  Partials       142      142

Impacted Files	Coverage Δ
samtranslator/model/s3_utils/uri_parser.py	71.42% <ø> (ø)	⬆️
samtranslator/model/apigateway.py	97.1% <ø> (ø)	⬆️
samtranslator/model/api/http_api_generator.py	98.11% <ø> (ø)	⬆️
samtranslator/model/sam_resources.py	94.06% <ø> (ø)	⬆️
samtranslator/model/api/api_generator.py	95.04% <50%> (ø)	⬆️

---

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[alexw91]

Codecov Report

Merging #1430 into develop will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1430   +/-   ##
========================================
  Coverage    94.37%   94.37%           
========================================
  Files           78       78           
  Lines         4678     4678           
  Branches       934      934           
========================================
  Hits          4415     4415           
  Misses         121      121           
  Partials       142      142           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7b47b87...60bbc2e. Read the comment docs.