Warn instead of error if ecm_install_icons finds no icons.

The V1 syntax of ecm_install_icons searched for icons by globbing files
with a particular naming pattern. If there were no such icons, this used
to do nothing, but silently. Commit fb7b8eea accidentally made this an
error. More sensible would be to make it a warning.

BUG: 354610
REVIEW: 125931