Use standardized API error format in tequilapi

[tadaskay]

Problem definition

• No consistent error format among the services. Most of them are also not easy to parse and forward to the API consumers. Chained API calls are all concatenated to a single message, e.g. server returned error: Possible error: another server returned error: you get a very long sausage: where the useful message is: in the very end.
• No consistent format for field validation that is easy to use by API consumers.

Solution

• Use APIError format defined in mysteriumnetwork/go-rest;
• Use apierror.ErrorHandler middleware for Gin to forward APIErrors (that have been added to gin.Context in request scope using c.Error()) to the caller and wrap any other type of errors. Returned content type is application/vnd.mysterium.error+json;
• Use apierror.Parse(*http.Response) *APIError to parse the incoming error response body based on Content-Type.

Scope of this PR

• Implemented for identities, payments endpoints in Tequilapi all Tequilapi endpoints and requests.Client
• Support this format in CLI

Related info

• APIError format: https://github.com/mysteriumnetwork/go-rest/blob/master/apierror/apierror.go
• Adaptation of go-rest in pilvytis: https://github.com/mysteriumnetwork/pilvytis/pull/492
• Adaptation of go-rest in mysterium-vpn-js: Change API error format to standard go-rest mysterium-vpn-js#119

Sample (with field validation):

{
  "error": {
    "code": "validation_failed",
    "message": "Request validation failed",
    "detail": "Request validation failed: passphrase: 'passphrase' is required [required]",
    "fields": {
      "passphrase": {
        "code": "required",
        "message": "'passphrase' is required"
      }
    }
  },
  "status": 400,
  "path": "/identities"
}

[codecov-commenter]

Codecov Report

Merging #4920 (e3bcb0a) into master (8aba570) will decrease coverage by 0.03%.
The diff coverage is 24.88%.

@@            Coverage Diff             @@
##           master    #4920      +/-   ##
==========================================
- Coverage   39.21%   39.17%   -0.04%     
==========================================
  Files         357      356       -1     
  Lines       19214    19018     -196     
==========================================
- Hits         7535     7451      -84     
+ Misses      10948    10845     -103     
+ Partials      731      722       -9     

Impacted Files	Coverage Δ
requests/client.go	70.37% <0.00%> (+8.07%)	⬆️
session/pingpong/hermes_promise_settler.go	34.03% <0.00%> (ø)
tequilapi/client/client.go	3.44% <0.00%> (+0.05%)	⬆️
tequilapi/contract/common.go	0.00% <0.00%> (ø)
tequilapi/contract/connection.go	0.00% <0.00%> (ø)
tequilapi/contract/entertainment.go	0.00% <0.00%> (ø)
tequilapi/contract/identity.go	0.00% <0.00%> (ø)
tequilapi/contract/mmn.go	0.00% <0.00%> (ø)
tequilapi/contract/pageable.go	0.00% <0.00%> (ø)
tequilapi/contract/session.go	0.00% <0.00%> (ø)
... and 34 more

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