ciao-controller: Return a meaningful error when workload does not exist.
ciao-controller: Return a meaningful error when workload does not exist.

When running "ciao-cli instance add -workload dummy"
the sql query fails reporting an error that looks like:

ciao-cli FATAL: HTTP Error [500] for [POST https://controller:8774
/v2.1/eb279e5a3ee846eaa88e5c035415ab42/servers]: sql: no rows in result set

This commits change the error message to let the user know that the
provided workload does not exists.

ciao-cli FATAL: HTTP Error [500] for [POST https://controller:8774
/v2.1/eb279e5a3ee846eaa88e5c035415ab42/servers]: Workload "dummy" not found

Fixes #292

Signed-off-by: Alberto Murillo <3e496b67a60a13551c6a151e0a6a52025fbd95e6@intel.com>

obedmr/ciao,leoswaldo/ciao,wcwxyz/ciao,jorgevgut/ciao,mrkz/ciao,tpepper/ciao,matthewrsj/ciao,markdryan/ciao,tpepper/ciao,01org/ciao,jorgevgut/ciao,mcastelino/ciao,ciao-project/ciao,mrkz/ciao,kaccardi/ciao,tpepper/ciao,erick0z/ciao,markdryan/ciao,kaccardi/ciao,obedmr/ciao,wcwxyz/ciao,matthewrsj/ciao,erick0z/ciao,mcastelino/ciao,ciao-project/ciao,rbradford/ciao,leoswaldo/ciao,jorgevgut/ciao,obedmr/ciao,mrkz/ciao,erick0z/ciao,leoswaldo/ciao,matthewrsj/ciao,wcwxyz/ciao,rbradford/ciao,mcastelino/ciao,01org/ciao