fetchart: Use success coloring if album art is found

[somasis]

This changes the 'has album art' message to use the green color used for success. I just think it makes it easier on the eyes.

[sampsyo]

Hmm... I like the notion, but it's also currently useful to have the cases where beets actually did new work to find album art stand out in the log. If everything but failure looks green, this is harder to see. Maybe we should be using a different color?

[somasis]

Blue? Yellow?

[sampsyo]

Good question; it's of course very subjective. 😃 Maybe yellow, since we already use that for an intermediate state? Or lightgray since we sort of want it to fade into the background?

[somasis]

How's that? lightgray (or, text_highlight_minor) sounded good to me.

[sampsyo]

Looks great! Thank you!