Make fish install a command_not_found handler in non-interactive uses.

Previously, fish's command_not_found handler would be installed in
__fish_config_interactive. Errors that occured early in startup (e.g. in
config.fish) or in non-interactive mode would therefore not be reported.
With this change, fish now exposes its default cnf handler as
__fish_default_command_not_found_handler . config.fish then installs a
cnfh that invokes the default. When fish goes interactive, the initial
cnfh is overwritten with a fancier one, that may in turn fall back to
invoking the default.

share/config.fish

@@ -10,6 +10,21 @@
 
 set -g IFS \n\ \t
 
+#
+# Create the default command_not_found handler
+#
+function __fish_default_command_not_found_handler
+    echo "fish: Unknown command '$argv'" >&2
+end
+
+#
+# Hook up the default as the principal command_not_found handler
+# This is likely to be overwritten in __fish_config_interactive
+#
+function __fish_command_not_found_handler --on-event fish_command_not_found
+    __fish_default_command_not_found_handler $argv
+end
+
 #
 # Set default search paths for completions and shellscript functions
 # unless they already exist