Preserve ConversionException cause

When improving the error message of the ConversionException thrown by the ConversionHandler in AbstractConfiguration.java:1768 the cause is lost.

If for example a string is converted into a URL the MalformedURLException, that contains the actual reason for the failed conversion, is gone.

A possible fix for that could be to attach the cause to the new ConversionException:

AbstractConfiguration.java:1768:

throw new ConversionException(
        String.format(
                "Key '%s' cannot be converted to class %s. Value is: '%s'.",
                key, cls.getName(), String.valueOf(value)), cex.getCause());