graph/education: slightly improve error handling and logging

- Use var for common errors
- Add the addition error message to the Error() output of errorcode.Error
- in PatchEducationSchool() use errorcode.RenderError() to turn the errorcode
  in to the right HTTP Status (instead of return 500 always)

services/graph/pkg/service/v0/errorcode/errorcode.go

@@ -130,7 +130,11 @@ func (e ErrorCode) String() string {
 }
 
 func (e Error) Error() string {
-	return errorCodes[e.errorCode]
+	errString := errorCodes[e.errorCode]
+	if e.msg != "" {
+		errString += ": " + e.msg
+	}
+	return errString
 }
 
 // RenderError render the Graph Error based on a code or default one