Fix spot fleet request panic

[aeschright]

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Handle errors on the spot fleet request resource that happen when an instance store-backed AMI is specified.

Note: the included test is skipped because it fails for an unrelated reason.

Closes #9814
Relates #9775

Release note for CHANGELOG:

NONE

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccAWSSpotFleetRequest_WithInstanceStoreAmi'
--- FAIL: TestAccAWSSpotFleetRequest_WithInstanceStoreAmi (108.34s)
    testing.go:629: Error destroying resource! WARNING: Dangling resources
        may exist. The full state and error is shown below.
        
        Error: errors during refresh: Instance store backed AMIs do not provide a root device name - Use an EBS AMI
        
        State: <nil>
FAIL
FAIL	github.com/terraform-providers/terraform-provider-aws/aws	109.126s

[aeschright]

Confirmed the fix with local testing:

Error: Instance store backed AMIs do not provide a root device name - Use an EBS AMI

  on main.tf line 75, in resource "aws_spot_fleet_request" "foo":
  75: resource "aws_spot_fleet_request" "foo" {

[nywilken]

Slight changed requested but otherwise good to go. Approving in advance.

[ghost]

This has been released in version 2.29.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!