fix: improve RequestFailedException error message

[JeyJeyGao]

Improve the error message:

Previous:

notation sign notationreg.azurecr.io/hello-app:v2 --plugin azure-kv --id https://acrci-test-kv.vault.azure.net/keys/self-signed-pkcs13/70747b2064c0488e936eba7a29acc4c6
Warning: Always sign the artifact using digest(@sha256:...) rather than a tag(:v2) because tags are mutable and a tag reference can point to a different artifact than the one signed.
Error: describe-key command failed: failed to execute the describe-key command for plugin azure-kv: ERROR: A certificate with (name/id) self-signed-pkcs13/versions/70747b2064c0488e936eba7a29acc4c6 was not found in this key vault. If you recently deleted this certificate you may be able to recover it using the correct recovery command. For help resolving this issue, please see https://go.microsoft.com/fwlink/?linkid=2125182
Status: 404 (Not Found)
ErrorCode: CertificateNotFound

Content:
{"error":{"code":"CertificateNotFound","message":"A certificate with (name/id) self-signed-pkcs13/versions/70747b2064c0488e936eba7a29acc4c6 was not found in this key vault. If you recently deleted this certificate you may be able to recover it using the correct recovery command. For help resolving this issue, please see https://go.microsoft.com/fwlink/?linkid=2125182"}}

Headers:
Cache-Control: no-cache
Pragma: no-cache
x-ms-keyvault-region: eastus
x-ms-client-request-id: 94abafcf-db8e-4046-be8a-573360b627eb
x-ms-request-id: b6c9de19-768c-4187-90d3-17342827af20
x-ms-keyvault-service-version: 1.9.1222.3
x-ms-keyvault-network-info: conn_type=Ipv4;addr=167.220.255.20;act_addr_fam=InterNetwork;
X-Content-Type-Options: REDACTED
Strict-Transport-Security: REDACTED
Date: Mon, 29 Jan 2024 06:52:00 GMT
Content-Length: 372
Content-Type: application/json; charset=utf-8
Expires: -1

Current:

notation sign notationreg.azurecr.io/hello-app:v2 --plugin azure-kv --id https://acrci-test-kv.vault.azure.net/keys/self-signed-pkcs13/70747b2064c0488e936eba7a29acc4c6   
Warning: Always sign the artifact using digest(@sha256:...) rather than a tag(:v2) because tags are mutable and a tag reference can point to a different artifact than the one signed.
Error: describe-key command failed: failed to execute the describe-key command for plugin azure-kv: CertificateNotFound: A certificate with (name/id) self-signed-pkcs13/versions/70747b2064c0488e936eba7a29acc4c6 was not found in this key vault. If you recently deleted this certificate you may be able to recover it using the correct recovery command. For help resolving this issue, please see https://go.microsoft.com/fwlink/?linkid=2125182

Resolves part of notaryproject/notation#868
Signed-off-by: Junjie Gao junjiegao@microsoft.com

[shizhMSFT]

LGTM