[Icons] report issues in ux:icons:warm-cache

[dbu]

when the http client is not available and an iconify icon is not already in the cache, the command does not indicate there is a problem:

single -v still almost no indication of problems, except that the other icon in my templates is not mentioned.

double -vv mentions the other icon and then has no "warmed" message for it, but does not explicitly state there was a problem

i would expect the command to fail, or at least output a fat warning even without extra verbosity that some icons where not found (and a check for HttpClient that mentions its absence)

[kbond]

This is a tricky one. The logic to find icons in your project looks at any string that matches something:something (even if it's not in a ux_icon()). The reason for this is to catch icons set in twig variables (ie ux_icons(error ? 'mdi:error' : 'mdi:success'). I'd like to improve this but not sure how...

In #2678, I've at least listed the not found icons when running ux:icons:warm-cache -v. Right now, I think that's the best we can do.