resource/aws_main_route_table_association: Prevent crash on creation when VPC main route table association is not found (#16680)
resource/aws_main_route_table_association: Prevent crash on creation when VPC main route table association is not found (#16680)

Reference: https://github.com/hashicorp/terraform-provider-aws/issues/16593

This is quick fix to replace the crash behavior with an actual error message. Additional information is needed to determine why this issue occurs (e.g. EC2 eventual consistency, problematic configurations, etc.) but this at least gives operators a better chance to continue other parts of the apply successfully and potentially just rerun the errant resource.

Output from acceptance testing:

```
--- PASS: TestAccAWSMainRouteTableAssociation_basic (62.60s)
```
terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws