Trying to load a non-existent structure writes exception type to log

The bug

When trying to load a non-existent structure the exception type is written to the log file. Since a missing file is something which can be expected, a simple error message like "The structure file %s does not exist" or similar should be sufficient. For all other exceptions the stack trace should be written to the log since it could be useful when trying to find the reason why an exception was thrown.