Add exceptions for invalid weight and length units

exceptions/updates/MDInvalidLicense.php

@@ -12,7 +12,7 @@
  * Custom exception class to be raised when trying to use a license that is not
  * accepted by museum-digital.
  */
-class MDInvalidLicense extends Exception {
+class MDInvalidLicense extends MDgenericInvalidInputsException {
 
     /**
      * Error message.