SYN-5006: Http error handling fix and add retry

[Pajk]

Resolves #68
Closes #67

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Based on PR #67

• do the same but for more resources
• make the linter happy

Pull request checklist

• Acceptance Tests have been updated, run (make testacc), and pasted in this PR (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Acceptance Test Output

go test ./synthetics/... -run TestAccCreateUpdateApiCheckV2                               
ok      github.com/splunk/terraform-provider-synthetics/synthetics      0.470s
go test ./synthetics/... -run TestAccCreateUpdateBrowserCheckV2                             
ok      github.com/splunk/terraform-provider-synthetics/synthetics      0.410s
go test ./synthetics/... -run TestAccCreateDowntimeConfigurationV2
ok      github.com/splunk/terraform-provider-synthetics/synthetics      0.485s
go test ./synthetics/... -run TestAccCreateUpdateHttpCheckV2 
ok      github.com/splunk/terraform-provider-synthetics/synthetics      0.478s
go test ./synthetics/... -run TestAccCreateLocationV2                                    
ok      github.com/splunk/terraform-provider-synthetics/synthetics      0.431s
go test ./synthetics/... -run TestAccCreateUpdatePortCheckV2                       
ok      github.com/splunk/terraform-provider-synthetics/synthetics      0.298s
go test ./synthetics/... -run TestAccCreateUpdateVariableV2                              
ok      github.com/splunk/terraform-provider-synthetics/synthetics      0.312s

Does this introduce a breaking change?

• Yes
• No

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