Provide function to obtain the unique element of a list
Provide function to obtain the unique element of a list

This version of 'the' properly lives in the 'Result' monad,
as opposed to traditional one calling 'error'. The reason
why it is 'Bad' that not precisely one element is returned
is given as argument.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>

mbakke/ganeti,yiannist/ganeti,leshchevds/ganeti,leshchevds/ganeti,yiannist/ganeti,andir/ganeti,yiannist/ganeti,grnet/snf-ganeti,ganeti/ganeti,dimara/ganeti,ganeti/ganeti,grnet/snf-ganeti,apyrgio/ganeti,onponomarev/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,leshchevds/ganeti,bitemyapp/ganeti,onponomarev/ganeti,dimara/ganeti,apyrgio/ganeti,andir/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,bitemyapp/ganeti