changes to new pattern substitution replacement; documentation updates

execute_cmd.c

@@ -5964,6 +5964,11 @@ shell_execve (command, args, env)
 	  errno = i;
 	  file_error (command);
 	}
+      else if (i == ENOENT)
+	{
+	  errno = i;
+	  internal_error (_("%s: cannot execute: required file not found"), command);
+	}
       else
 	{
 	  /* The file has the execute bits set, but the kernel refuses to