MM-27916: Improve logging when session is not found (#15292)

* MM-27916: Improve logging when session is not found

The error handling in this code is pretty bad and the same error message happens
for multiple conditions, making it difficult to diagnose the real issue.

Most of the times, we get a log like:

```
"Invalid session        {"error": "GetSession: Invalid session token=jodb6sau47rnugaqj1fy7khmpr, err=<no value>, "}"
```

And it could have happened from multiple places. So this log turns out to be not that useful.

We improve this by populating the Error field to fix the "<no value>" issue and also add a separate detailed error
field for each log line to uniquely identify each error.

* fix

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/session.go

@@ -54,7 +54,7 @@ func (a *App) GetSession(token string) (*model.Session, *model.AppError) {
 		if session, nErr = a.Srv().Store.Session().Get(token); nErr == nil {
 			if session != nil {
 				if session.Token != token {
-					return nil, model.NewAppError("GetSession", "api.context.invalid_token.error", map[string]interface{}{"Token": token, "Error": ""}, "", http.StatusUnauthorized)
+					return nil, model.NewAppError("GetSession", "api.context.invalid_token.error", map[string]interface{}{"Token": token, "Error": ""}, "session token is different from the one in DB", http.StatusUnauthorized)
 				}
 
 				if !session.IsExpired() {
@@ -74,13 +74,15 @@ func (a *App) GetSession(token string) (*model.Session, *model.AppError) {
 			if err.Id != "app.user_access_token.invalid_or_missing" {
 				detailedError = err.Error()
 				statusCode = err.StatusCode
+			} else {
+				mlog.Warn("Error while creating session for user access token", mlog.Err(err))
 			}
-			return nil, model.NewAppError("GetSession", "api.context.invalid_token.error", map[string]interface{}{"Token": token}, detailedError, statusCode)
+			return nil, model.NewAppError("GetSession", "api.context.invalid_token.error", map[string]interface{}{"Token": token, "Error": detailedError}, "", statusCode)
 		}
 	}
 
 	if session == nil || session.IsExpired() {
-		return nil, model.NewAppError("GetSession", "api.context.invalid_token.error", map[string]interface{}{"Token": token}, "", http.StatusUnauthorized)
+		return nil, model.NewAppError("GetSession", "api.context.invalid_token.error", map[string]interface{}{"Token": token, "Error": ""}, "session is either nil or expired", http.StatusUnauthorized)
 	}
 
 	if *a.Config().ServiceSettings.SessionIdleTimeoutInMinutes > 0 &&
@@ -103,7 +105,7 @@ func (a *App) GetSession(token string) (*model.Session, *model.AppError) {
 					mlog.Warn("Error while revoking session", mlog.Err(err))
 				}
 			})
-			return nil, model.NewAppError("GetSession", "api.context.invalid_token.error", map[string]interface{}{"Token": token}, "idle timeout", http.StatusUnauthorized)
+			return nil, model.NewAppError("GetSession", "api.context.invalid_token.error", map[string]interface{}{"Token": token, "Error": ""}, "idle timeout", http.StatusUnauthorized)
 		}
 	}