perl.c: Improve buildcustomize error message

buildcustomize.pl was being loaded with generated code that died if
‘lib/buildcustomize.pl’ existed but do-file failed.  $@ was used as
the error message.

If the file does not exist (do-file searching @INC), then $@ is empty
and the error is in $!.  Before 9054c817, this is what we would get:

$ ./miniperl -T -Ilib -e0
Died.
BEGIN failed--compilation aborted.

I fixed that in 9054c817 so there would be no error (by adding an ini-
tial ‘./’ to the path.)

That message was rather baffling.  It would have been easier to track
down the cause if I had seen something like this:

$ ./miniperl -T -Ilib -e0
lib/buildcustomize.pl: No such file or directory.
BEGIN failed--compilation aborted.

This commit changes the generated code to output $! like that if $@ is
empty, in case a similar bug occurs again.