Improve failure message

src/assert.bash

diff --git a/src/assert.bash b/src/assert.bash
index a52454a..178e299 100644
--- a/src/assert.bash
+++ b/src/assert.bash
@@ -336,8 +336,9 @@
   # Matching.
   if (( is_mode_empty )); then
     if [ -n "$output" ]; then
-      echo 'expected no output, but output was non-empty' \
-        | batslib_decorate 'unexpected output' \
+      batslib_print_kv_single_or_multi 6 \
+          'output' "$output" \
+        | batslib_decorate 'output non-empty, but expected no output' \
         | fail
     fi
   elif (( is_mode_regexp )); then

test/50-assert-16-refute_output.bats

diff --git a/test/50-assert-16-refute_output.bats b/test/50-assert-16-refute_output.bats
index 10c8fc4..35e6fa9 100755
--- a/test/50-assert-16-refute_output.bats
+++ b/test/50-assert-16-refute_output.bats
@@ -37,8 +37,8 @@
   run refute_output
   [ "$status" -eq 1 ]
   [ "${#lines[@]}" -eq 3 ]
-  [ "${lines[0]}" == '-- unexpected output --' ]
-  [ "${lines[1]}" == 'expected no output, but output was non-empty' ]
+  [ "${lines[0]}" == '-- output non-empty, but expected no output --' ]
+  [ "${lines[1]}" == 'output : a' ]
   [ "${lines[2]}" == '--' ]
 }