label generator bugs for specimen and aliquot page

https://catissueplus.atlassian.net/browse/PLUS-1001: Error displayed on
UI if user not enter the barcode on add specimen page.
https://catissueplus.atlassian.net/browse/PLUS-996: Validation message
not displayed if user enter the already existed Label/barcode
https://catissueplus.atlassian.net/browse/PLUS-999: Aliquot showing auto
generated for label field when Label Generator is off