albumartcache: Logger warning format incorrect

Use correct string formatting call. Convert the error code into the enum
name.
3 jobs for wip/mschraal/artcache-error-reporting in 21 minutes and 56 seconds (queued for 21 minutes and 7 seconds)
Status Job ID Name Coverage
  Check
passed #7620
pep8

00:04:54

passed #7619
pyflakes

00:04:50

 
  Build
passed #7622
build

00:12:11