pattern: error out in case of reference loop

This changes behavior, we used to ignore the loop and use the
default. However both Firefox and Chrome error out in this case
so let's return a rendering error.
7 jobs from paint-source in 1 minute and 53 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
failed #177914
fedora test

00:01:53

 
  Lint
skipped #177916
allowed to fail manual
clippy
skipped #177915
rustfmt
skipped #177917
tarball
 
  Cross Distro
skipped #177920
debian_i386:testing
skipped #177919
debian:testing
skipped #177918
fedora:rawhide
 
Name Stage Failure
failed
fedora test Test
error: Could not compile `unicode-normalization`.

To learn more, run the command again with --verbose.
make[2]: *** [Makefile:1954: /builds/pborelli/librsvg/_build/target/debug/librsvg_internals.a] Error 101
make[2]: Leaving directory '/builds/pborelli/librsvg/_build'
make[1]: *** [Makefile:1460: all-recursive] Error 1
make[1]: Leaving directory '/builds/pborelli/librsvg/_build'
make: *** [Makefile:963: all] Error 2
ERROR: Job failed: exit code 1