return bad request instead of server error for identity group cycle detection

[ccapurso]

Providing member_group_ids that contains a cycle for an identity group to identity/group/name/<name> will currently result in a 500 response. This should instead be a 400 response stating that a cycle was detected.

Reproduction steps:

Create groups:

ID_Group_0=`vault write -format=json identity/group/name/Group_0 | jq -r ".data.id"
ID_Group_1=`vault write -format=json identity/group/name/Group_1 | jq -r ".data.id"
ID_Group_2=`vault write -format=json identity/group/name/Group_2  | jq -r ".data.id"

Specify group IDs for Group_0 and Group_1:

vault write identity/group/name/Group_0 member_group_ids= 
vault write identity/group/name/Group_1 member_group_ids=$ID_Group_2 

Specify group IDs for Group_2 will fail due to cycle:

vault write identity/group/name/Group_2 member_group_ids=$ID_Group_0,$ID_Group_1 

Error writing data to identity/group/name/Group_2: Error making API request.

URL: PUT http://127.0.0.1:8200/v1/identity/group/name/Group_2
Code: 500. Errors:

1 error occurred:
	* failed to perform cyclic relationship detection for member group ID "c5ac421e-0748-8b7e-466b-5a951542c2f8"

This PR introduces improved error handling such that an error caused by a cycle detection can be handled more specifically by a request handler. This allows for returning a logical.ErrorResponse rather than bubbling up the error to Vault's response handling logic and resulting in a 500 as so:

vault write identity/group/name/Group_2 member_group_ids=$ID_Group_0,$ID_Group_1 

Error writing data to identity/group/name/Group_2: Error making API request.

URL: PUT http://127.0.0.1:8200/v1/identity/group/name/Group_2
Code: 400. Errors:

* cyclic relationship detected for member group ID "752b6146-2cc3-b3f5-8ab6-9feea4da98b5"

[ccapurso]

It looks like some tests failed, looking into those.

[hghaf099]

Asked a question, otherwise looks good!