Fix a lot of linting issues

After we introduced the linter we haven't really touched the old code.
This commit now fixes all linting issue that exist right now.

api/environments.go

@@ -2,6 +2,7 @@ package api
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"github.com/gorilla/mux"
 	"gitlab.hpi.de/codeocean/codemoon/poseidon/api/dto"
@@ -15,6 +16,8 @@ const (
 	createOrUpdateRouteName   = "createOrUpdate"
 )
 
+var ErrMissingURLParameter = errors.New("url parameter missing")
+
 type EnvironmentController struct {
 	manager environment.Manager
 }
@@ -35,7 +38,7 @@ func (e *EnvironmentController) createOrUpdate(writer http.ResponseWriter, reque
 
 	id, ok := mux.Vars(request)[executionEnvironmentIDKey]
 	if !ok {
-		writeBadRequest(writer, fmt.Errorf("could not find %s", executionEnvironmentIDKey))
+		writeBadRequest(writer, fmt.Errorf("could not find %s: %w", executionEnvironmentIDKey, ErrMissingURLParameter))
 		return
 	}
 	environmentID, err := runner.NewEnvironmentID(id)
@@ -54,8 +57,3 @@ func (e *EnvironmentController) createOrUpdate(writer http.ResponseWriter, reque
 		writer.WriteHeader(http.StatusNoContent)
 	}
 }
-
-// delete removes an execution environment from the executor
-func (e *EnvironmentController) delete(writer http.ResponseWriter, request *http.Request) { // nolint:unused ToDo
-
-}