provider/aws: Allow recovering from failed CloudWatch Event Target creation

[radeksimko]

Closes #5389

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=AWSCloudWatchEventTarget'

TF_ACC=1 go test ./builtin/providers/aws -v -run=AWSCloudWatchEventTarget -timeout 120m
=== RUN   TestAccAWSCloudWatchEventTarget_basic
--- PASS: TestAccAWSCloudWatchEventTarget_basic (22.41s)
=== RUN   TestAccAWSCloudWatchEventTarget_full
--- PASS: TestAccAWSCloudWatchEventTarget_full (94.04s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    116.482s

[stack72]

LGTM!

[ghost]

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