CORE-2451 cloud_storage_clients/abs_clients: fix iterator check

[andijcr]

fixed a missing error_code_it != headers.end() check

Normally on Azure, the HTTP 400 error response has a x-ms-error-code header, so headers.find("x-ms-error-code")->value() is well-behaved, but it's not safe to not check the iterator before accessing it.

Fixes CORE-2451

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x

Release Notes

• none

[andijcr]

manually tested this matrix of config

cloud_storage_credentials_source = [azure_vm_instance_metadata]
cloud_storage_azure_hierarchical_namespace_enabled = [None, True, False]
azure container backend =[with HNS, without HNS]

some of the combinations result in ERRORS:

cloud_storage_azure_hierarchical_namespace_enabled = True, azure container backend = without HNS
cloud_storage_azure_hierarchical_namespace_enabled = False, azure container backend = with HNS

and redpanda correctly handles the exceptions without crashing

tested successfully with missing x-ms-error-code with dev...andijcr:redpanda:followup/aks_oidc_support

[andijcr]

2 Known failures: #17176
#13841