resource/aws_kms_grant: Properly return error when listing KMS grants

[bflad]

Fixes #5062

Changes proposed in this pull request:

• Properly return ListGrants errors from findKmsGrantById() instead of crashing
• Additionally prevent potential nil dereferencing panics by using SDK methods in findKmsGrantById()

Output from acceptance testing:

$ make testacc TEST=./aws TESTARGS='-run=TestAWSKmsGrant'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAWSKmsGrant -timeout 120m
=== RUN   TestAWSKmsGrant_Basic
--- PASS: TestAWSKmsGrant_Basic (53.14s)
=== RUN   TestAWSKmsGrant_withConstraints
--- PASS: TestAWSKmsGrant_withConstraints (82.90s)
=== RUN   TestAWSKmsGrant_withRetiringPrincipal
--- PASS: TestAWSKmsGrant_withRetiringPrincipal (52.32s)
=== RUN   TestAWSKmsGrant_bare
--- PASS: TestAWSKmsGrant_bare (52.20s)
=== RUN   TestAWSKmsGrant_ARN
--- PASS: TestAWSKmsGrant_ARN (51.72s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	292.322s

[tombuildsstuff]

LGTM 👍🏻

[bflad]

This has been released in version 1.27.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

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