__fish_print_help: proper error message when 'man' is missing

Commit 3f2e4b71bc (functions/__fish_print_help: bravely remove
fallback to mandoc/groff, 2025-11-09) savagely made commands like
"abbr -h" require man, or else it prints a noisy stack trace.
Some packages have not yet replaced "nroff/mandoc" with "man"
as dependency, and, independent of that, man is still an optional
dependency.

Make missing "man" a proper error, which is easier to read tan the
stack trace.

share/functions/__fish_print_help.fish

@@ -1,5 +1,10 @@
 # localization: skip(private)
 function __fish_print_help --description "Print help for the specified fish function or builtin"
+    if not type -q man
+        fish_command_not_found man
+        return 1
+    end
+
     set -l item (__fish_canonicalize_builtin $argv[1])
 
     function __fish_print_help_man -V item -a man1