tests/test-debug-flags.sh: Log the error from nbdkit unconditionally.

Useful to print the error displayed by nbdkit in all cases, not
just when the test fails.

Also fix the error message if the test does fail (unexpected -> expected).

tests/test-debug-flags.sh

@@ -49,10 +49,9 @@ expected_failure ()
 
 check_error ()
 {
+    cat debug-flags.out
     if ! grep -sq "$1" debug-flags.out; then
-        echo "unexpected error message containing: $1"
-        echo "actual output:"
-        cat debug-flags.out
+        echo "$0: expected error message not present in above output: '$1'"
         exit 1
     fi
 }