LUDelTags: Improve formatting of error message
LUDelTags: Improve formatting of error message

Use utils.CommaJoin to add spaces after comma, clean up code a bit.

Before: Tag(s) 'bar','baz','foo','moo' not found
After: Tag(s) 'bar', 'baz', 'foo', 'moo' not found

Signed-off-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

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