Ensure we have an import error before using in import path

[stack72]

Without this, we can panic.

Fixes: #5883
Fixes: #5860
Fixes: #5876
Fixes: pulumi/pulumi-aws#1252

Previous behaviour:

     Type                 Name                 Plan
 +   pulumi:pulumi:Stack  import-post-dev      create.
 =   └─ aws:iam:Policy    AdministratorAccess  import.
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x4a80f56]

goroutine 73 [running]:
github.com/pulumi/pulumi/pkg/v2/resource/deploy.(*deploymentExecutor).importResources(0xc000eefa60, 0x5c10020, 0xc00151c4e0, 0x869c2a8, 0xc0005b2dc0, 0x7fffffff, 0x0, 0x0, 0x0, 0x0, ...)
	/Users/runner/work/pulumi/pulumi/pkg/resource/deploy/deployment_executor.go:459 +0x276
github.com/pulumi/pulumi/pkg/v2/resource/deploy.(*deploymentExecutor).Execute(0xc000eefa60, 0x5c10020, 0xc00151c4e0, 0x869c2a8, 0xc0005b2dc0, 0x7fffffff, 0x0, 0x0, 0x0, 0x0, ...)
	/Users/runner/work/pulumi/pulumi/pkg/resource/deploy/deployment_executor.go:138 +0x1247
github.com/pulumi/pulumi/pkg/v2/resource/deploy.(*Deployment).Execute(...)
	/Users/runner/work/pulumi/pulumi/pkg/resource/deploy/deployment.go:383
github.com/pulumi/pulumi/pkg/v2/engine.(*deployment).run.func1(0x5c1ac60, 0xc0005b2dc0, 0xc0005b3180, 0x5c10020, 0xc00151c4e0, 0xc00151c401, 0xc000e91f10, 0xc0000a3aa0)
	/Users/runner/work/pulumi/pulumi/pkg/engine/deployment.go:251 +0x267
created by github.com/pulumi/pulumi/pkg/v2/engine.(*deployment).run
	/Users/runner/work/pulumi/pulumi/pkg/engine/deployment.go:237 +0x306

New behaviour:

     Type                 Name                 Plan       Info
 +   pulumi:pulumi:Stack  import-post-dev      create     1 error
 =   └─ aws:iam:Policy    AdministratorAccess  import     1 error

Diagnostics:
  pulumi:pulumi:Stack (import-post-dev):
    error: preview failed

  aws:iam:Policy (AdministratorAccess):
    error: Preview failed: refreshing urn:pulumi:dev::import-post::aws:iam/policy:Policy::AdministratorAccess: 1 error occurred:
    	* Error reading IAM policy AdministratorAccess: InvalidParameter: 1 validation error(s) found.
    - minimum field size of 20, GetPolicyInput.PolicyArn.