lib.fileset.toSource: Improve error for unknown file types

This does decrease performance unfortunately

Benchmarking expression toSource { root = ./.; fileset = ./.; }
Mean CPU time 0.103747 (σ = 0.012415) for 10 runs is 97.32181384964636% (σ = 16.34179537413021%) of the old value 0.106602 (σ = 0.0125571)
Statistic .envs.elements (205920) is 105.5842% (+10891) of the old value 195029
Statistic .gc.totalBytes (20247696) is 101.7495% (+348160) of the old value 19899536
Statistic .nrThunks (134824) is 108.7878% (+10891) of the old value 123933
Statistic .symbols.number (996) is 100.1005% (+1) of the old value 995
Statistic .values.number (275238) is 104.1199% (+10891) of the old value 264347

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/fileset/tests.sh b/lib/fileset/tests.sh
index 86ef1989f60b5..5b756b8fc5931 100755
--- a/lib/fileset/tests.sh
+++ b/lib/fileset/tests.sh
@@ -356,7 +356,9 @@ rm -rf -- *
 
 # non-regular and non-symlink files cannot be added to the Nix store
 mkfifo a
-expectFailure 'toSource { root = ./.; fileset = ./a; }' 'file '\'"$work"'/a'\'' has an unsupported type'
+expectFailure 'toSource { root = ./.; fileset = ./a; }' 'lib.fileset.toSource: `fileset` contains a file that cannot be added to the store: '"$work"'/a
+\s*This file is neither a regular file nor a symlink, the only file types supported by the Nix store.
+\s*Therefore the file set cannot be added to the Nix store as is. Make sure to not include that file to avoid this error.'
 rm -rf -- *
 
 # Path coercion only works for paths