abbr -e: use same exit code as set -e if abbr doesn't exist

Which is 4, apparently.. (builtin_set.cpp returns ENV_NOT_FOUND)
here. This was previously hardcoded to our 121, which used to be
what builtins used for invalid arguments.

4 is pretty arbitrary but at least this is more consistent.

share/functions/abbr.fish

@@ -102,7 +102,7 @@ function __fish_abbr_erase --no-scope-shadowing
 
     if not set -q $abbr_var_name
         printf ( _ "%s %s: No abbreviation named %s\n" ) abbr --erase $escaped_name >&2
-        return 121
+        return 4 # like `set -e doesnt_exist`
     end
 
     set -e $abbr_var_name